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Photoresponsive nanostructure assisted green synthesis of organics and polymers
Accepted Manuscript Title: Photoresponsive Nanostructure Assisted Green Synthesis of Organics and Polymers Authors: Maosong Liu, Tingyu Peng, Henan Li, Long Zhao, Yuanhua Sang, Qunwei Feng, Li Xu, Yinhua Jiang, Hong Liu, Jianming Zhang PII: DOI: Reference:
S0926-3373(19)30195-X https://doi.org/10.1016/j.apcatb.2019.02.071 APCATB 17506
To appear in:
Applied Catalysis B: Environmental
Received date: Revised date: Accepted date:
15 October 2018 31 January 2019 25 February 2019
Please cite this article as: Liu M, Peng T, Li H, Zhao L, Sang Y, Feng Q, Xu L, Jiang Y, Liu H, Zhang J, Photoresponsive Nanostructure Assisted Green Synthesis of Organics and Polymers, Applied Catalysis B: Environmental (2019), https://doi.org/10.1016/j.apcatb.2019.02.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoresponsive Nanostructure Assisted Green Synthesis of Organics and Polymers Maosong Liu,a Tingyu Peng,a Henan Li,a Long Zhao,a Yuanhua Sang,b Qunwei Feng,c Li Xu,d
a
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Yinhua Jiang,a Hong Liub,e* and Jianming Zhanga*
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013,
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China.
State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China.
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Jiaxing Rejdue Environmental Technology Co. Ltd, Jiaxing, 314006, China.
Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu
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d
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b
Institute for Advanced Interdisciplinary Research (IAIR), University of Jinan, Jinan 250022,
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e
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University, Zhenjiang, 212013, China.
China.
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Graphical abstract
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Corresponding author email: [email protected], [email protected]
Highlights
Review of recent advances in heterogenous photocatalytic organic synthesis. 1
Photoresponsive nanostructure assisted conversion of CO2 to organics.
Heterogenous photocatalytic organic conversion.
Photoresponsive nanostructure initiated radical polymerization.
Nanoparticle initiators for photosynthesis.
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Abstract
Solar light is believed to be the most sustainable and clean energy source. In line with the
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concept of green chemistry, the use of solar energy has been a formidable impetus toward the
development of novel photo-based synthetic technologies to drive various chemical reactions. Photosynthesis is a promising route to achieve a wide array of chemical transformations with
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distinctive energy and environmental merits which are usually inaccessible with conventional
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thermal processes. Photoinduced organic synthesis, as one important branch of photosynthesis,
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has attracted increasingly attentions to meet the growing-up environmental and energy concerns.
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Highly photoresponsive nanoparticles (NPs), such as semiconductors, can generate photoexcited charge carriers, i.e. electron-hole pairs, upon photon absorption, which can favor
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a vast number of reactions, demonstrating unique advantages including the easily recycling and
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reuse, the use of mild reaction conditions and the generation of high-purity products devoid of contaminants. This review article highlights recent representative advances in heterogenous
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photocatalytic organic synthesis, mainly including CO2 reduction, organic transform and free radical polymerization; in particular, for the first time, the achievements on photoresponsive NP initiated free radical polymerizations are summarized here. We conclude this review by
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proposing several interesting research directions and future challenges with the hope that it can serve as a good reference for researchers in nanomaterials and catalysis. Keywords: Heterogenous photocatalysis, CO2 reduction, Photo organic synthesis, Photo polymerization, Nanoparticles 1 Introduction 2
Today, synthetic organic chemistry constitutes a crucial part of human endeavors in chemical science, which offers us materials in huge diversity, such as fuels, biomolecules and plastics. Typically, to obtain desired chemicals with high production yield or drive synthesis at high reaction rate, harsh reaction / operation conditions, e.g. high temperature and / or pressure, are usually necessary in many synthetic processes, which usually are driven by fossil energy. The
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over-consumption of the fossil fuels has caused excessive emission of carbon dioxide (CO 2) and other toxic byproducts, which produce serious pollutants to the environment. The
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increasing environmental and sustainable-development requirements have thus impelled chemists to explore advanced “green” and clean routes for organic synthesis [1–4]. The ecosystem on the Earth is initiated and supported by the super power source of the sun,
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almost without exception. Upon the acceptance of the solar energy by plants and other light-
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harvesting organisms, chemical transitions are initiated to form new chemicals to make
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creatures vital and diverse. Such a “green” and clean process, known as the natural
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photosynthesis, utilizes sunlight and carbon dioxide (CO2)/water as a sustainable energy source
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and cheap raw materials, respectively (Figure 1 left). Inspired by the natural photosynthesis of plants, artificial photochemistry technology has been intensely investigated for chemical
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synthesis [5–10]. In principal, a photoreaction is triggered by a photon-electron-transfer (PET) process, where upon photon energy absorption electrons at molecule orbitals enter the excited
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states and become highly active to promote chemical transformations. Compared with its thermal counterpart, photosynthesis demonstrates a distinctive advantage to meet the energetic
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requirements in conducting reactions. Using incident light as energy source, a large variety of chemical reactions can be initiated to selectively catalyze chemical transformations which would be difficult to be activated thermally [5–10]. Solar energy is without contest the greenest and most sustainable energy source currently accessible. Therefore, photochemistry based on solar light is a sustainable, eco-friendly chemical process which has attracted significant attention. Using solar energy to directly convert CO2 and water into carbon-containing fuels or 3
chemicals is a major topic and frontier in synthetic chemistry. Based on the recognition of the photo-responsive molecules, such as organic dyes or transition metal chromophores, photoinduced organic synthesis from small molecule to polymer scale has been established and rapidly developed for more than a century [5–10]. Nonetheless, many drawbacks still exist in
molecules are toxic and may remain in products as impurities [11,12].
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the photo-responsive molecule-initiated synthesis; for example, many of the photo-responsive
Initiated by the seminal work of Fujishima on 1979 [14], the research of heterogenous
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photocatalysis has led to fruitful achievements, and is still in a rapid growth stage. Numerous
types of photoresponsive nanoparticles (NPs) have been created and intensely studied [13–21]. As illustrated in Figure 1 (right), in typical heterogenous photocatalysis, the absorption of
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photons excites electrons from valence band (VB) to conduction band (CB) of the
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semiconductor NP (e.g. TiO2), leading to the generation of electron-hole pairs (e-/h+) or charge
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carriers. The so-generated electron-hole pairs immigrate to the surface of the NP to interact
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with electron acceptors and donors adsorbed on NP surface via the PET process, forming
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extremely active reducing and oxidizing species respectively. The synthesis, therefore, can be proceeded through either oxidative or reductive pathways, or through the combination of both
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[13,19]. From view point of green chemistry, semiconductor NPs show desirable properties as photoreaction initiators [13,16,18], where the structures and components can be intendedly
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designed and tuned, using a plethora of known methods, to obtain better charge separation and higher quantum efficiency. More interestingly, compared with the homogeneous photoreactions
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using small molecules (e.g., dyes), NPs are more chemically stable, environmentally benign as well as less toxic. In addition, as the NPs are insoluble in reaction solution, they can be easily recycled / reused in the successive reactions without remaining in the products, thus leading to a generation of clean products in “one-pot” reactions [13–21]. The pioneer work by Inoue [14] using semiconductor as photocatalyst has opened a revolutionary venue to realize the conversion of CO2 into simple carbon fuels or organics, e.g. 4
HCOOH, CH3OH, HCHO and CH4, upon solar light as the sustainable energy source, revealing a promising route to reduce greenhouse gas and simultaneously produce an array of useful organics in a green fashion and leading to a vast number of publications. Significant achievements have been obtained in the evolution of reaction efficiency as well as the understanding of the reaction mechanism. Some excellent reviews have summarized the-state-of-the-art of heterogenous
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photocatalytic CO2 reduction applications [22–28]. For instance, Gong et al. [25] reviewed the main strategies to improve the surface interaction (adsorption/activation) of CO2 molecules on
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semiconductors and the factors dominating the reaction pathways. Jaroniec et al. [26] summarized
various 3D hierarchical photocatalysts including nano / microspheres, hetero-structured nanocomposites, yolk/shell structures and hollow structures etc., bearing the advantages of large
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surface areas, high CO2 adsorption capabilities and high mass transfer rates for catalysis. Zhang et
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al. [29,30] reviewed the CO2 reduction using perovskite oxide nanomaterials, and pointed that
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perovskite oxides exhibit wider absorption, longer charge carrier lifetime, and favorable band-
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alignment for CO2 reduction, which is supposed to be more active than conventional semiconductors.
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Yu et al. [31] have reviewed the application of graphene (GR) materials in photocatalytic CO2 reduction, showing that due to the unique properties including excellent conductivity, good optical
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transmittance, large specific surface area, and superior chemical stability, GR-based catalysts demonstrate good activity. The daily-updated reports enrich the fundamental understanding as well
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as the development of highly efficient catalysts. Although abundant scientific results in this domain, currently heterogenous photocatalytic
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CO2 reduction is mainly applied to produce small hydrocarbons. To produce more complex or value-added organics, heterogenous photocatalytic organic conversion has been developed. Upon the presence of photo-generated active radicals, redox reactions can be conducted to small organic molecules, such as alcohols or alkenes, to produce corresponding derivates or new chemicals under a mild reaction condition. Some reviews on photocatalytic organic conversion have been published [23,24,40,41,32–39], of which several are more specific on certain 5
reactions. For example, in a review by Yao et al. [37], photocatalytic oxidation reactions, especially, alcohols oxidation, aromatic hydroxylation and alkene oxidation are summarized. Xu and Luque et al. [38,42] reviewed the advances in the selective heterogenous photocatalytic valorisation of lignin-based compounds. Li et al. [24] surveyed the work on oxidation of amines, carbon-carbon bond formation and carbon-hetero bond formation. Bahnemann and co-workers
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[23] reviewed the reaction selectivity in photocatalytic organic synthesis, and pointed that the screening of photocatalysts for specific reactions may reach better selectivity. Nonetheless, the
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fundamental understanding on the exact reaction mechanism is still lacking.
Polymers or macromolecules can also be obtained via a heterogenous photocatalytic process. Very recently,
heterogenous photocatalytic polymerization technique has
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demonstrated an evolution to its traditional thermal counterpart to meet the growing demand
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for easy curing, highly functionalized and well-defined polymer structures as building blocks
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in nanotechnology, biological and 3D fabrication applications [43,44]. A conventional free
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radical polymerization process usually works at high temperature, where initial radicals are
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generated by thermal decomposition of molecule initiators (e.g. organic/inorganic peroxides or diazo compounds) or upon reacting redox-active compounds [45–48]. Despite their remarkable
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ease of use, their low cost and wide availability, these initiators also show several drawbacks due to their residual presence in the polymer, their lack of sustainability, their odor and potential
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toxicity. Heterogenous semiconductor NPs show desirable properties as photoinitiators [13,16,18], since the photogenerated charges can result in the formation of highly active radicals
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to initiate the reaction. However, compared with the intense focus of CO2 reduction and organic conversion, the study on heterogenous photocatalytic polymerization receives less attention, and the review with a systematic summary of the advances in this research domain is missing. As a complementary to previous excellent reviews, here we have reviewed some up-todate reports with high novelties from the most prestigious journals. We mainly introduce the application of NP catalyzed photoradical organic synthesis. We start by highlighting the recent 6
advances of semiconductor NPs for photocatalytic CO2 reduction and move to organic conversion, including alcohols oxidation, aromatic hydroxylation and so on. To differentiate from other review articles as mentioned above, we mainly introduce some very recent state-ofthe-art achievements and give a summary of the key parameters affecting the efficiency and/or selectivity of reactions, which are highly diverse yet have been strongly concerned in this field.
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Then we move to the section of photoinduced free radical polymerization with more detailed
information in a relatively longer length, which is systematically summarized for the firstly
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time; in particular, latest achievements on the use of inorganic NPs for controlled or living radical photopolymerizations are introduced. The fundamental mechanisms underlying the
reactions are also briefly addressed. This review concludes with challenges and proposals on
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how to broaden the utilization of photoresponsive NP in organic transforms, in particular for
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polymerization, and how to gain more fundamental understanding on the mechanism of
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photosynthesis. We expect that this review could be a valuable reference for the readers who
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are interested in photocatalytic synthesis for environmental and clean energy generation.
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2. Photoresponsive NP Catalyzed Organic Synthesis 2.1 Photocatalytic Conversion of CO2 to Organics
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As aforementioned, reports about heterogenous photocatalytic CO2 reduction have been
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emerging numerously. Nowadays, to obtain high performance, most of the photocatalysts have been evolved into hetero or hybrid nanostructures, which are mainly composed of several different components with multi-functions or rationally designed architectures. Simple and
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classic semiconductors, such as TiO2 or CeO2 NPs, are seldom used alone for efficient photocatalysis due to their intrinsic drawbacks, e.g. low absorption and quantum yield; however, they are still active for mechanism study and employed as good platforms to fabricate more complex structures. In this section, the content is sequenced on the basis of catalyst type. We start from the most classic TiO2 by reviewing several latest innovations of simple 7
semiconductor system; afterward we introduce some recent advances of modified semiconductors and nanohybrids. At the end, the parameters affecting reaction efficiency are discussed briefly. 2.1.1 Simple semiconductor catalysts (TiO2, CeO2 and so on) TiO2 NPs, as the most intensively investigated photocatalyst, have been widely applied to
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catalyze the reduction of CO2 into fuels and hydrocarbons because of their excellent UV light
absorption, chemical stability, low cost and easy fabrication etc. [49]. Through precisely
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programming the microstructures of TiO2 NPs, such as porosity, oxygen vacancy or crystallite
structure, an idea platform can be created to investigate the structure–activity relationship to gain more fundamental understanding on catalysis. Oxygen vacancies (OVs) have been
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acknowledged to play a vital role in many photocatalytic reactions [50–52]. The OVs at surface
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or bulk show distinct functions for catalysis [50–52]. To better understand the effect of OV on
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catalysis, Yang et al. [53] prepared three different TiO2 NPs with surface OVs (SOVs) and/or
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bulk single-electron-trapped OVs (SETOVs) via simple approaches of dehydration or reduction
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of different Ti precursors. Photocatalytic CO2 reduction test under Hg lamp irradiation showed that both the bulk SETOVs and SOVs contribute to light absorption enhancement. The SOVs
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facilitate charge-carrier separation, while the bulk SETOVs work more as the recombination center. For the case of the co-existence of the surface and bulk OVs, the catalytic efficiency to
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convert CO2 to CH4 can be improved by increasing the ratio of surface OVs to bulk SETOVs, indicating a synergistic effect for catalysis. The presence of OV-related defects could not only
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act as the activation sites for catalysis [53–55], but also can enhance the light absorption of TiO2 to the visible range [56]. Using KIT-6 SiO2 as template, Russo et al. [57] synthesized mesoporous TiO2 NPs with high surface area (190 m2/g vs. 53 m2/g of P25). CO2 reduction tests showed that the catalysis upon this mesoporous TiO2 led to more hydrocarbons, better reaction kinetics and higher stability, due to its superior characteristics than those of the conventional TiO 2 NPs. The larger 8
surface area of mesoporous TiO2 is highly beneficial to lower adsorbing energy barrier of reactants (CO2 and H2O) to TiO2. A recent study using mesoporous TiO2 NP for CO2 reduction [49] proposed that that upon photon absorption, photo-excited electrons reduce the adsorbed CO2 to an anion radical CO2•−, while H2O oxidation is preceded through the interaction with holes, leading to the formation of H+ and OH• radicals, as demonstrated in Figure 2. H+ may
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accept electrons to form H•, which then reacts with CO2•− to form CO, CH•, CH2• and CH3•.
Subsequently, these alkyl radicals interact with OH• to form CH3OH as a product with a
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combination of CH4 and O2 [24]. The reaction proceeds mainly by the competitive adsorption
of CO2 and H2O on the surface of TiO2 NPs. In addition, UV light intensity, H2O/CO2 ratio, and structure of NPs all exert important impact on the performance of the catalyst and product
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yield. The saturation of adsorption center may induce an undesired reverse reaction, where the
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produced methanol reacts with oxygen (O2) to generate CO2 [49]. Kang and co-workers [58]
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also synthesized ordered mesoporous metal oxides (TiO2 and SnO2) with high surface areas
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(~100 m2/g) for CO2 photoreduction. Owing to their high surface area, the mesoporous metal
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oxides demonstrate 9 to 10 times higher yields of CH4 and 2 to 3 times higher yields of CO than P25.
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Rossetti et al. [59] studied the mechanism of photoreduction of CO2 at high pressure in
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liquid phase using P25 as photocatalyst in the presence of Na2SO3 as hole scavenger working at different pH. Using a special reactor which allows improving CO2 solubility in water at high pressure, products of liquid phase (formic acid, formaldehyde) and gas phase (CH 4) can be
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successfully produced. Reaction time investigation showed that at the beginning, liquid product of formaldehyde was the main product, followed by the formation of formic acid. Interestingly, as the extension of reaction time, liquid phase products reduce gradually (Figure 3, upper), while gases, mainly composed of H2 and CO, start forming and become the major product after the consumption of the Na2SO3, which can be ascribed to the photodegradation of the pre9
formed organic products (Figure 3, lower). At this stage, the organic species formed at the beginning, e.g. formaldehyde, may act as alternative hole scavengers. By optimizing reaction condition, such as temperature, pH and reaction time, the productivity of formaldehyde and formic acid can reach as high as 16.537 mol h−1 kgcat−1 and 2.95437 mol h−1 kgcat−1, respectively. Recent study shows that anatase TiO2 nanocubes with single crystalline and exposed {100}
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and {001} facets, synthesized by hydrothermal and calcination approach, display significantly
high photoactivity in CO2 reduction to CH4 and CH3OH, because of the synergetic effects of
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highly ordered crystallization, co-exposal of {100} and {001} and more negative CB position
facets (Figure 4 a and b) [60]. CeO2 NPs have also been broadly used as photocatalyst in CO2 reduction. A novel CeO2 homojunction nanomaterial with unique crystallite structures has been
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fabricated via a crystallographic-oriented growth method by Zhou and co-workers [61], as
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shown in Figure 4 c-f, where hexahedron prism with exposed {100} facet was anchored on
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octahedronal with exposed surface of {100} facet. The inert CeO2 octahedron can be activated
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by prism arms on the surfaces, leading to an increase of its photocatalytic activity for the
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reduction of CO2 into CH4. Three possible factors were proposed to interpret the significantly enhanced performance: first, the prism arms act as fast channels for the transportation of the
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photogenerated electrons of the homojunction; second, the band alignment of the two exposed facet of {100} and {111} favors the charge separation to the surface of the catalyst; third, high
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charge carrier mobility due to the different effective mass of electrons of {100} and {111} facets greatly facilitates the charge separation.
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2.1.2 Modified semiconductors In addition to simple metal oxide NPs, compound semiconductors, such as g-C3N4, ZnS, CdS and CdSe have also been used for photocatalytic CO2 reduction [23,24,32–41,62–69,70–75]; however, the photoactivity is restricted due to their deficient inherences, such as bandgap limit or high charge recombination rate. Therefore, a great number of efforts have been conducted to
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increase the absorption and enhance the quantum efficiency of conventional semiconductors, including doping, dye-sensitizing and forming heterostructures or cocatalysts. 1) Doped semiconductors The introduction of dopant, either metals or non-metal elements into semiconductor lattice can effectively suppress the recombination of photogenerated charge carriers, rapidly transfer
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electrons/holes to the adsorbed reactants, and correct the band gap structure, even by the
concomitant red shift of the intrinsic absorption edge. For instance, doping TiO2 with N, S and
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Ni2+ [76–78] can narrow the band gap and thus increase the visible light absorption. Yu et al. [79] synthesized S-doped g-C3N4 with wider absorption range up to 475 nm. Photocatalytic
CO2 reduction test showed that the CH3OH production yield using S-doped g-C3N4 is much
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bigger than using undoped g-C3N4 (1.12 vs. 0.81 μmol g−1). Recently Yu also reported O-doped
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g-C3N4 nanotubes as photocatalyst for CO2 reduction with improved performance [80]. Besides
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the modification to band gap and charge separation efficiency, the O-doping to g-C3N4 enhances
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the affinity and uptake capacity to CO2. TiO2 NPs doped with Indium (In) exhibits increased
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surface area and enlarged band gap [81]. The photocatalytic efficiency of In-doped TiO2 NPs was greatly improved for CO2 reduction. CO was the main product over pure TiO2, whereas
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CH4 yield was considerably increased using In-doped TiO2. This enhancement is mainly attributed to the largely increased interfacial transfer efficiency of photogenerated charges,
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leading to effective charge separation and suppressed recombination of photogenerated charge carriers, owing to the structural change induced by the insert of In into TiO 2 lattice. Ola and
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Maroto-Valer [82] reported that the Cu doped TiO2 show visible light absorption with enhancing Cu loading concentration in comparison with pure TiO2. This change is ascribed to the interaction of Cu+ ions with TiO2 crystal lattice via the replacing Ti4+ with Cu+; moreover, these Cu+ species can act as electron traps to suppress the charge recombination and favor the multielectron reactions, thus largely improve the CO2 reduction efficiency. Other transition
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metal elements (such as Fe, Co, Pt, Mo and Ni etc.) doped TiO2 are also found to be effective photocatalysts for reducing CO2 to CH4, and other hydrocarbons [81–92]. 2) Sensitized semiconductor The activity of semiconductor NPs can also be enhanced by sensitizing their surface with photoresponsive molecules, e.g. organic or metal complex dyes, to surface of the
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semiconductors. In this system, the photogenerated electrons can transfer between sensitizer
and CB of semiconductor, and the transfer pathway is determined by the relative positions of
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the highest occupied molecular orbital and lowest unoccupied molecular orbital
(HOMO/LUMO) levels of dyes and CB of semiconductors. In most cases, the dye molecule acts as electron donor and semiconductor as electron acceptor during photocatalysis [93]. The
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semiconductor with only UV light response, such as TiO2 or ZnO, can be easily modified into
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a visible-light responsive one using suitable sensitizers, Metal complex molecules are more
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stable than organic dyes, and they are thus widely used as sensitizers in photocatalysis. As
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presented in Figure 5, Pawan et al. [94] reported a Ru(bpy)3Cl2 sensitized TiO2 nanohybrid
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(Ru(bpy)3/TiO2) as highly selective catalyst for photocatalytic reduction of CO2 under visible light. In this system, Ru(bpy)32+ component absorbs visible light and transfer electrons to the
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CB of TiO2 for reaction. It is found that the visible light-driven methanol production yield upon Ru(bpy)3/TiO2 hybrid catalyst was much higher than that using pure TiO2 catalyst and
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Ru(bpy)3Cl2 alone. P25 has also been sensitized using Cu complex of Cu(II) tetra(4carboxylphenyl)porphyrin (CuTCPP) using a hydrothermal approach [95]. CH4 production in
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potocatalytic CO2 reduction using CuTCPP/P25m (0.5 wt % of CuTCPP) is approximately 46 times higher than that using P25 alone, which is mainly attributed to the Cu sensitizer, which enhances the absorption of visible light and the rate of charge carrier separation. In another report [33], a comparison study using different sensitizers of Cu(II) phthalocyanines (CuPc) and Cu(II) porphyrins showed that the Cu(II) phthalocyanine(CuPc) (Cu(II) co-ordinated in the middle of the macrocycles) is highly beneficial for the photoactivity, probably due to its 12
favorable reduction potential, indicating the important role of sensitizer structure on catalytic activity. Currently many attentions have been focused on phthalocyanines containing center metals including Zn(II), Cu(II), Co(II), Ni(II) as sensitizers of semiconductors [96,97]. 3) Heterostructures or hybrids Without doubt, today almost entirely of efforts in this research domain are concentrated to
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rationally design semiconductor heterostructures or nanohybrids to improve the reaction
efficiency. The CO2-reduction heterostructure catalysts can be generally divided into three
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categories: noble metal-semiconductor hybrid catalysts, noble metal-free hybrid catalysts, and
multi-semiconductor hybrid catalysts, among which most of the attention are focused on the noble metal-semiconductor heterostructures, leading to the huge increase of publications daily.
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As there have been some excellent reviews about this topic, here we mainly summarize some
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a) Noble metal-based hybrid catalyst
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very recent and most representative work.
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Noble metal NPs, such as Pt [98–104], Pd [105,106], Ag [107–110], Ru [111,112], Au [113]
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and alloy (or bimetallic) NPs [114–119] have been broadly applied to conjugated with semiconductors to form heterostructures. The synthesis to integrate metal NPs with
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semiconductor can be generally divided into two methods, which are the in-situ growth of metal along with semiconductor, and the coupling of as-synthesized components. Composition,
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morphology, structure, crystal facets, size, and alloy phase of the noble metal NPs demonstrate profound effect on the activity and selectivity of photocatalytic CO 2 reduction.
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Among all noble metals, Pt possesses the unique feature of the highest work function of
5.56 eV, suggesting a very low Fermi level and strong electron-accepting ability. Therefore, Pt nanostructures have been largely applied to decorate semiconductor surface to promote separation of photo charge carriers, thus enhancing catalytic efficiency [28]. Pt NP decorated TiO2 (Pt/TiO2) is the most intensively investigated system for photocatalytic CO2 reduction. For example, very recently, Tesbihi and Fresno [102] reported a mesoporous SiO2 supported 13
Pt/TiO2 catalyst, which was synthesized by classic deposition-precipitation method. CO is the main product with only tracible CH4 and CH3OH using TiO2 alone as the photocatalyst, while using Pt/TiO2, CH4 becomes the major product with a selectivity of ~100% with optimum Pt loading. In-situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) characterization indicates that the Pt NPs on TiO2 can strongly chemisorb the CO intermediate,
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thus preventing CO to be the main product and further promoting the total reduction of CO 2 to CH4. Zhang et al. [103] loaded Pt and Cu2O on TiO2 surface for CO2 reduction. In the hybrids,
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Pt usually tends to promote the production of CH4 with undesired H2. On the other hand, Cu2O is an effective material to suppress the production of H2; however, it shows lower CH4
selectivity than that of Pt. Thus, the combination of both with an appropriate ratio can largely
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improve CH4 production. It is found that Pt shows dual functions to accelerate separation and
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transport of photogenerated electrons, and increase the electrons density of Cu2O NPs, which
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is beneficial for selective CH4 formation. In another report of Pt/In2O3 system, researchers [104]
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found that due to electronic and steric effect, Pt NPs can modulate the surface feature of In2O3,
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reducing the formation of bicarbonates (HCO3−), bidentated carbonate (b-CO32−) and monodentated carbonate (m-CO32−) species toward the production of HCOOH, CH2O and
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CH3OH. More importantly, Pt play a key role in the selective formation of CH 4, as it weakens H-Pt bond and facilitate the bonding of C in CO2 with protons to form CH4, which
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simultaneously restrains CO production. As the most studied catalyst, Pt has been widely employed to couple with various semiconductors, such as g-C3N4, CeO2, Zn2GeO4, LaPO4
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[61,99,120,121], and composites of In2O3/g-C3N4, NaNbO3/g-C3N4, HNb3O8/SiO2, CdSe/TiO2, CdS/SiO2 etc [74,100,101,122,123], to improve reaction activity and selectivity. Pd has also been decorated to semiconductors to improve reaction efficiency of photocatalytic CO2 reduction. Through a density functional theory (DFT) study, Liu et al. [124] had proposed that Pd/In2O3 is a promising catalyst with high activity for CO2 reduction to methanol, because Pd is expected to improve the catalysis by offering metal sites for H2 14
dissociation-adsorption and interfacial sites for CO2 adsorption-hydrogenation, though there is still challenge experimentally. Very recently, they have synthesized the Pd/In2O3 hybrid via a thermal decomposition method, which successfully prevented the formation of PdIn alloy at composite interface [125]. Highly-dispersed small Pd-NPs are exposing predominately the {111} facets on In2O3 surface. The Pd NPs demonstrate better ability to adsorb–dissociate
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hydrogen, thereby providing active H for the hydrogenation and accelerating the generation of oxygen vacancies on semiconductor surface. The Pd-In2O3 interfacial sites are also vital to
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promote CO2 adsorption and hydrogenation. As a result, a superior performance of the Pd/In2O3
catalyst for CO2 reduction to methanol is reached with a CO2 conversion >20% and methanol selectivity >70%. Bai et al. [105] investigated the relationship between Pd NP facet and
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catalytic selectivity, by loading single-crystal Pd nanoclusters (NCs) or Pd nanotubes (NTs)
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enclosed by {100} and {111} facets on g-C3N4 nanosheets. The Pd/g-C3N4 samples all showed
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enhanced charge separation efficiency; however, they found that the Pd NT/g-C3N4 showed a
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much higher selectivity (80%) than that of Pd NC/g-C3N4 (20%) in CO2 reduction to CO,
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C2H5OH, and CH4. This facet selectivity relationship is attributed to the higher adsorption energy (EA = 0.23 eV) and lower activation energy barrier (EB = 3.98 eV) on Pd {100} than
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those on Pd {111}, respectively.
Au, Ag and Cu NPs are well-known materials as cocatalyst for visible light driven
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photocatalytic water splitting, due to their unique optical property of surface plasmon resonance (SPR). These plasmonic nanohybrids have also been used for CO2 reduction. Au NP is deemed
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to be the most famous SPR material with strong and tunable visible to near infrared absorption. Various Au/TiO2 photocatalysts are used for CO2 reduction under visible light irradiation [126,127]. For example, mesoporous Au@TiO2 yolk−shell hollow spheres with highly crystalline and thin TiO2 has been reported by Zou’s group [127]. The SPR-induced local electric field at Au/TiO2 not only enhances the separation of photo charge carriers in TiO2 shells to improve the CO2 reduction rate but also favors the formation of high-grade carbon species 15
(C2H6), which is rarely reported in CO2 reduction systems. They also reported the Ag/TiO2 catalyst for efficient CO2 reduction owing to the SPR and electron sinker effect of Ag component, which is closely related with the amount and size of loaded Ag. In this system, diverse hydrocarbons were produced, mainly including CH4 and methanol [127]. Cu NPs have also been hybridized on TiO2 surface (Cu/TiO2) to increase the production yield of CH3OH, as
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they exhibit a dual role as electron sink to improve the photo-charge separation and light
harvesters to enhance the visible light absorption due to their localized SPR [128,129]. Besides
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the SPR property, these noble metals also show catalytically active sites in reactions. Small Ag
NPs have been loaded on ZnGa2O4, ZnTa2O6, SrNb2O6 and Sr2Nb2O7 for photoreduction of CO2 [130–132]. Tanaka et al. [131] proposed the reaction mechanism on Ag/SrNb2O6 nanorod,
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where the reduction of CO2 led to the formation of CO mainly on the top of the Ag/SrNb2O6
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nanorods, while O2 was generated mainly on the sides of the nanorods. The separation of the
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reduction and oxidation sites greatly suppressed the recombination of the photogenerated
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charge carriers; Ag-loaded SrNb2O6 nanorods thus exhibit good activity and selectivity. In
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another report, Vajda el al. [133] found small Cu clusters on Fe2O3 surface favor the activation of hydrogen, which can spill over to the neighboring Fe sites, and thus enhance the conversion
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CO2. Other Ag based compound catalyst, such as AgBr NPs have also been used for efficient CO2 reduction [134].
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Recently, Ru has attracted increasing attentions as a promising replacement to expensive
noble metals (i.e. Pt), as they show good activity and stability if rationally designed and more
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importantly price advantage compared with Pt [112,135–137]. For example, Zhang et al. [135] studied the effect of different oxide support (Al2O3, CeO2, MnO2 and ZnO) of Ru based catalysts on CO2 reduction. For the pristine oxides alone, CO was the major product due to a lack of H2 dissociation sites on these oxides. The addition of Ru created H2 dissociation sites and led to a significant increase of CO2 conversion and CH4 selectivity. The ZnO support resulted in the formation of lowest Ru-CO coverage with a weak CO adsorption and the reverse 16
water-gas shift was the dominant reaction. CeO2 showed a moderate Ru-CO coverage in between Al2O3 and ZnO, which favors the creation of H2 dissociating sites and remains the high Ru-CO adsorption strength. Zeng et al. [136] reported a mesoporous silica nanowire encapsulated Ru NP catalyst (Ru@mSiO2) for CO2 reduction. CO was the main product for 13 nm Ru NPs within mesoporous silica nanowires, while CH4 was the major one for bigger Ru
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NPs (5-20 nm). This difference is mainly ascribed to the different reaction intermediates formed on Ru@mSiO2 catalyst surface: CO-Run+ on 1–3 nm and formate species on 5–20 nm.
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Bimetallic NPs, including alloys, core-shell and intermetallic structures, usually contains at least one noble metal, which have demonstrated the so-called synergistic effect on promoting the photocatalytic CO2 reduction. In most cases, these bimetallic NPs are distributed on
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semiconductor surface as cocatalysts for photoreactions. For example, Song et al. [138]
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reported a SiO2 supported Pd-Cu alloy NP catalysts for selective CO2 hydrogenation to
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methanol. In the alloy structure, the component ratio of Pd could be varied in the range of 25–
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34 mol %. It was found that the alloy with 25 mol % Pd (Pd(0.25)–Cu/SiO2) demonstrated the
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highest activity with a methanol formation rate twice higher than the simple sum of monometallic Cu and Pd catalysts, indicating a synergistic effect for catalysis. It is proposed
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that the alloys could provide a source of chemisorbed hydrogen atoms, which favors the formation of CH3OH. Zhao and co-workers [116] reported a three-dimensionally ordered
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microporous (3DOM) TiO2 supported Au-Pd core-shell-like NPs. Since bimetallic AuPd nanoparticles with the relatively low Fermi level have good capacity of trapping electron, they
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can efficiently promote the separation of photogenerated electron-hole pairs in TiO2. The AuPd/3DOM-TiO2 catalysts demonstrate excellent activity for photocatalytic CO2 reduction. In particular, Au3Pd1/3DOM-TiO2 catalyst exhibits the highest activity and selectivity for CO 2 reduction to CH4 with a formation rate of 18.5 μmol g -1 h-1 and selectivity around 93.9 %. In photocatalysis, upon light absorption, the excited electrons in CB of TiO2 are transferred to Au NPs. And then the electrons further transfer from Au to Pd, which further decreases the 17
recombination rates of electron-hole pairs. Electrons assemble on metal NPs react with the adsorbed CO2 to form CH4, while holes on the VB of TiO2 oxidize H2O. Therefore, Au-Pd coreshell NPs not only offers the reduction sites but also serves as the electron sinks, leading to an effective charge separation and photocatalytic activity enhancement. AuFe, AuAg and Pd-In based catalysts have also been reported and show large activity enhancement for CO2 reduction
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into hydrocarbon products [118,119,139]. b) Noble metal free catalyst
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Compared with the noble metal-based catalyst, non-noble metal one exhibits superior cost
advantage, implying higher feasibility for mass-production or commercialization. Ni is a famous material as non-noble metal cocatalyst for various catalytic reactions, including CO2
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reduction [140–142]. For instance, Dai et al. [143] reported a Y2O3 supported Ni NP catalyst
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as photocatalyst for CO2 reduction. By changing different synthetic methods, the authors found
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the approach using YO(NO3) acting as a unique matrix to afford anchoring sites to interact with
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Ni2+ ions, can result in a moderate interaction between Ni metal and Y2O3 support, thus inducing
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excellent catalytic activity and stability towards CO poisoning in photocatalytic CO2 reduction. Garcia and Albero [130] synthesized GR supported NiO/Ni NPs (NiO/Ni-GR) as efficient
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photocatalyst for gas phase CO2 reduction with H2. The hybrid with an optimal Ni loading of 23 wt % showed the highest activity with the maximum specific CH4 formation rate (642 μmol
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gNi−1 h−1 at 200 °C). Under the same conditions, Ni NPs supported on SiO 2-Al2O3 or NiO NPs alone only showed much lower CH4 production rates than NiO/Ni-GR. In another report, the
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presence of Ni@NiO core@shell co-catalyst increased the photoactivity of N doped InTaO4 (N-InTaO4) for CO2 reduction to CH3OH under visible light irradiation [30]. Zou’s group [144] reported a KOH-modified Ni/LaTiO2N photocatalyst for visible light CO2 reduction, showing high performance in CO2 reduction with CH4 generation rate of 9.69 μmol g−1, about 5 times higher than that upon LaTiO2N. The significant activity enhancement is mainly attributed to a formation of Schottky barrier at Ni/LaTiO2N interface boosting the separation of electron-hole 18
pairs. The OH- of KOH promotes the activation of CO2 into CO32- species, greatly enhancing the reaction kinetics of CO2 reduction, and the OH- can also act as hole acceptor, accelerating the proton transfer from H2O oxidation. Metallic Cd has also been used as cocatalyst to couple with semiconductors for photocatalytic CO2 reduction. Inoue et al. [145] reported that the Cd decorated ZnS (Cd/ZnS) increased the reaction efficiency of photocatalytic reduction of CO 2 to
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HCOOH. c) Non-metallic (or metal free) hybrid catalyst.
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Although most efforts have been concentrated on the metal-based hybrid catalyst with superior activity, the cost is still very high, as those metals are limited in supply, and refining
process is less environmental-friendly. By stark contrast, the non-metallic nanohybrids, mainly
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composed of semiconductor-based (mainly oxides, compound semiconductors and g-C3N4),
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carbon based (e.g., GR, reduced graphene oxide (rGO), carbon nanotube (NT)) or metal-organic
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framework (MOF)-based materials show many excellent features, such as easy synthesis,
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highly designable morphologies, high surface area, good electric conductivity, nontoxic,
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inexpensive, and feasible for mass fabrication, leading to a promising material for photocatalytic CO2 reduction. For example, recently, Guo et al. [146] reported a g-C3N4@CeO2
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hollow heterostructure with rich OVs. This hybrid demonstrates a significant synergetic effect in catalysis. The OVs of g-C3N4 @CeO2 is found to play the vital role for CO2 reduction,
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resulting in the generation of CH4 much earlier with higher concentration than that upon pristine g-C3N4 and CeO2 alone. Yu et al. [96] reported a g-C3N4/ZnO structure, which exhibited
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stronger and wider light absorption than pure g-C3N4 in the visible range. The photocatalytic activity of this g-C3N4/ZnO for CO2 reduction to CH3OH is 2.3 times higher than that of pure g-C3N4. Rather than the conventional heterojunction-type mechanism, a direct Z-scheme mechanism was proposed to interpret the enhanced properties of the g-C3N4/ZnO system, where the intimate contact of the two components results in more efficient electron transfer from ZnO to gC3N4 without an electron mediator can take place at the g-C3N4/ZnO interface (Figure 6). The 19
mechanism is further confirmed by PL analyses, photocurrent measurements and DFT calculations. Kocí et al. [147] find kaolinite/TiO2 composite shows higher photoactivity than P25 for CO2 conversion to CH4, The enhanced performance is attributed to two key factors: (1) the decrease of anatase crystallite size when TiO2 NPs was introduced into the kaolinite structure;
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(2) the modification of acidobasic properties of catalyst surface inhibiting the charge
recombination and preventing the aggregation of TiO2 during reaction. Backov and Fécant [148]
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also reported novel SiO2-TiO2 self-standing porous foam with a size of millimeter length scale for 3D-photodriven CO2 catalytic reduction. The foams bear a strongly multidiffusive performance, in which photons can be greatly trapped within the cores by increasing the light
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pathway. This 3D process largely promotes selectivity for the hydrocarbon production of
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methane (~ 80%) and ethane (~ 18%). Olivo et al. [149] reported that N-doped CuO-TiO2
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photocatalyses CO2 photoreduction in gas phase under a very mild reaction condition. The study
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of N and Cu amount effect on catalytic performances showed that the CH4 formation is sensitive
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to the amount of Cu, and the reagent ratio also strongly affects the catalytic performance. Fang et al. [150] reported CuO-doped TiO2 hollow microspheres in light-driven CO2 conversion with
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H2O to CH4., and the crystal structure effect on CO2 conversion has also be investigated. Hollow spherical mesoporous TiO2 NPs possess large surface areas and layered nanostructures, with a
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large number of surface-active sites. The CuO clusters on TiO2 surface act effectively as cocatalyst to improve photocatalytic activity. Compared with the most prevailing photocatalyst,
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e.g. P25 TiO2, the TiO2 hollow microsphere catalyst hybridized with CuO exhibited higher photocatalytic activity for photo-driven reduction of CO2 with H2O into CH4. Cuprous oxide (CuxOy) nanoclusters coupled with binuclear units of ZrOCoII were covalently anchored on a silica mesopore surface to form a hierarchical assembly for CO2 reduction. The oxidation state of the Cu on surface can be tuned by the reduction of CuxOy cluster via a metal-to-metal charge transfer state of the ZrOCoII light absorber. The production yield of the HCOOH showed strong 20
dependence on the oxidation state of the surface Cu, indicating that the heterobinuclear unit functions as light absorber to donate electrons to the CuxOy catalyst to drive the CO2 reduction [151]. GR materials possess excellent conductivity and optical property, thus have been broadly applied in photocatalysis to enhance the efficiency of photo-charge separation and
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transportation. rGO modified TiO2 reveals improved activity for photoreduction of CO2 to CH4 under visible light, which is six-fold higher than the pure TiO2 [152]. Tu et al. [153] successfully
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fabricated robust hollow spheres which consist of molecular alternating Ti0.91O2 NPs and rGO nanosheets toward the photocatalytic conversion of CO2 in the presence of water vapor. The conversion of CO2 using rGO-Ti0.91O2 hollow spheres is 5-times higher than pure Ti0.91O2
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hollow spheres and 9-times higher than P25 [154,155], since i) the ultrathin nature of the
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material favors the rapid migration of charge carrier to surface, ii) the strong interaction of
incident
light
trapping
by
multi-scattering
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the
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Ti0.91O2 with rGO enhances the lifetime of charge carriers, and iii) the hollow structure increases effect.
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core-shell
structured
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rGO@CuZnO@Fe3O4 microspheres was fabricated by Jain et al [156], which is a highly efficient, recyclable and magnetically separable catalyst for CO 2 reduction under visible light
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to produce methanol. The excellent activity of this hybrid is ascribed to the restoration of the sp2 hybridized aromatic system of rGO, which promotes the movement of electrons and leads
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to efficient charge separation. The synthesized heterogenous photocatalyst could readily be collected by magnet and reused for several cycles without significant activity loss.
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MoS2-based nanomaterial has attracted wide attention as they possess a suitable band gap
for visible-light harvesting and large surface are for reactant adsorption, rendering them a promising catalyst for photocatalysis [157–159]. For example, very recently, Li and co-workers [158] synthesized a marigold-like nanoflower constructed by SiC@MoS2 nanosheets, which demonstrated excellent activity for CO2 reduction under visible light irradiation to form CH4 (Figure 7). The CH4 evolution efficiency was reported to reach the value of 323 μL·g-1·h-1 along 21
with nearly stoichiometric evolution of O2 under visible light without using any sacrificial reagents during reaction, which is a record for simultaneously visible-light driven photocatalytic CO2 reduction to CH4 and water oxidation to O2. This superior photocatalytic performance can be ascribed to (1) the Z-scheme heterostructure of the SiC@MoS2 nanosheets, which favors electron transfer upon more negative CB of SiC and more positive VB of MoS2;
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(2) the higher hole mobility of MoS2 and higher electron mobility of SiC. (3) the large surface
area of marigold flower-like morphology of SiC@MoS2; (4) the gas-solid reaction mode, which
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is beneficial for adsorption/desorption of reactants/products.
Bismuth based semiconductors, such as Bi2 WO6, BiOX (X = Cl, Br, I) and Bi2S3 are cheap materials, and show excellent photocatalytic activity due to their unique optical and structural
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features [160–168]. For example, Fu’s group [168] reported a Bi2 WO6-TiO2 binanosheet Z-
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scheme heterostructure toward high selectivity CH4 formation (Figure 8). They revealed that
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an interaction and a competition between light-driven carbonaceous residues decomposition
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and photocatalytic CO2 reduction are coexisted during reaction, suggesting the importance of
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removing organic residues from photocatalysts for an efficient CO2 reduction. Moreover, this work could provide some enlightenment on designing and/or synthesizing more efficient
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photocatalysts for CO2 reduction. BiOX photocatalysts possess unique layered crystalline structure and visible absorption property. Under visible/near-infrared-light, BiOI has been used
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as catalyst to convert CO2 into CO and CH4 [169]. Yu et al [170] developed a novel 2D/2D heterojunction of untrathin Ti3C2/Bi2 WO6 nanosheets, where Bi2 WO6 was grown along the
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surface of Ti3C2 nanosheet. This hybrid possesses a short charge transfer pathway and a large interfacial surface, leading to superior charge transfer performance. Moreover, the high surface area as well as high porosity enables Ti3C2/Bi2 WO6 hybrids enhanced CO2 adsorption capability. This 2D/2D hybrid demonstrated largely enhanced catalytic activity for photoreduction of CO2. The production yield of CH4 and CH3OH upon the hybrids is 4.6 times higher than that using pristine Bi2WO6 nanosheets. Bismuth sulfide (Bi2S3) is a p-type 22
semiconductor with a narrow band gap (1.49 eV), and the CB potential is more negative than that of most photocatalysts indicating a good visible light absorption. Recently, Bi2S3 nanoribbons with very high aspect ratio were synthesized and used for the photocatalytic reduction of CO2 to methanol [161]. Bi2S3 NPs were also used as co-catalyst to conjugate with other semiconductors, such as CeO2 [171] and CdS [172], to form nanocomposite catalyst for
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visible light CO2 reduction to CH3OH, and demonstrated dramatically enhanced activity, compared with the reactions catalyzed by each single component, which is attributed that the
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Bi2S3 co-catalyst effectively inhibits the electron-hole recombination and favors charge transfer. Since surface OVs are believed to play a decisive role in determining the efficiency of photocatalytic reactions, very recently Zhang and co-works [173] published an excellent review
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discussing about the OV-mediated photocatalysis at the surface molecular level using 2D
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BiOCl as the platform, indicating the importance of Bi based semiconductor in photocatalysis.
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Besides the inorganic materials, coordination polymers and MOF have been largely applied
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in photocatalytic CO2 reduction [174–181]. These organic metal complex materials provide a
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newly emerging platform to integrate light-harvesting and catalytic centres for photocatalysis due to their high diversity of structure and controllable synthetic procedures [150–155]. An
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artificial Z-scheme system of p-type InP/Ru complex polymer-TiO2 hybrid photocatalyst was introduced for the photoelectrochemical reduction of CO2 to formate. Upon this hybrid material,
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selective photoreduction of CO2 to HCOO− was achieved in aqueous media, in which H2O was used as both an electron donor and a proton source. The selective yield of HCOO− is more than
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70% and the conversion efficiency from solar energy to chemical energy is 0.03 ~ 0.04% [182]. Amino-functionalized NH2-UiO-66(Zr), MOF-253 supported active Ru carbonyl complex (MOF253–Ru(CO)2Cl2), amine-functionalized Ti-based porous MOF (NH2-MIL-125(Ti), (Ti8O8(OH)4(NH2-BDC)6, BDC=benzene-1,4-dicarboxylate) and other types of MOFs were synthesized and used for visible light photocatalytic CO2 reduction [40,175,183–186]. For example, Xu and co-workers [186] reported the application of a porphyrin-based MOF PCN23
222 in the photo organic synthesis. Their study showed that this MOF structure can selectively capture and further photo-reduce CO2 efficiently under visible light. The deep electron trap states present in the unique structure of PCN-222 effectively suppress the electron-hole recombination compared to the corresponding porphyrin ligands, leading to significantly enhanced photocatalytic conversion of CO2 to form formate anion. There are still many other
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types of photocatalysts in this research domain, such as Perovskite oxide and biological
catalysts, this information can be found in other excellent reports and reviews [25,27–30,187–
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190]. 2.1.3 Factors affecting products
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In general, the heterogenous photocatalytic CO2 reduction is started from the adsorption and
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activation of CO2 on catalyst surface; subsequently, PET from catalysts to the adsorbed CO2
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triggers chemical reactions which eventually determine the final products and the efficiency of
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the photocatalysis [191–193]. The process of adsorption and activation of CO2 on catalyst surface is the key step which significantly affects the following hydrogenation of CO2 and the
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suppression of competing H2 evolution reaction. Basically, CO2 adsorption induces the formation of partially charged species CO2•−, which present on catalyst surface in three modes
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[191–193]. If CO2•− is interacted with surface atoms of catalyst via O atom(s) (Figure 9 a), it
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prefers to take a H atom to form a formate anion (HCOO−) and then form formic acid (HCOOH) through protonation as a final product, where the C−O bond is stable and no cleavage occurs [194,195]. If the bonding is taken place between C atom of CO2•− (Figure 9 and c), CO2•− tends
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to combine with a H to form carboxyl radical (•COOH) and then split into the adsorbed CO, where the C−O bond breaks easily [195,196]. The adsorbed CO can be subsequently hydrogenated to form various hydrocarbons. Therefore, in principle, by the rational design of catalyst surface, the adsorption and activation of CO2 on catalyst surface may be regulated, leading to the formation of desired products [191–193]; however, a good control to the 24
adsorption process is a considerable challenge in practice and the understanding of the exact mechanism of catalysis is still limited. There are also many factors influencing the activity and selectivity of the catalysis, such as OVs, exposed facet, surface area (or porosity), heterojunctions and so on, which may show combined effects to reactions. In many cases, the results are unpredicted from the initial design. Although a clear law affecting the selectivity of
surface chemical and solution pH) are extracted from some published results.
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1) Cocatalyst effect
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the reaction is too complicate to be concluded, several important parameters (co-catalyst,
As mentioned above, today, nearly all efforts have been concentrated on development of heterostructures, among which the hybridization of semiconductor with cocatalysts dominates
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the research domain. The cocatalyst system can greatly improve the efficiency of photocatalytic
N
CO2 reduction, as it can essentially improve the surface reaction kinetics by decreasing the
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activation energy. A report of the CuO loaded P25 from Gunlazuardi et al. [197] showed a
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lower activation energy than P25 (23 vs. 26 KJmol-1), indicating that the presence of CuO
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enhanced the catalytic efficiency [197]. The Schottky barrier at the interface of metal NPsemiconductor can improve the charge separation leading to an electron accumulation at the
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metal side for catalysis [198]. The adsorption of CO intermediate on catalysis, the hydrogen evolution and CO2 activation overpotentials are the key factors determining the selectivity
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[199,200]. The metals, such as Pb, possessing high hydrogen evolution and CO 2 activation overpotentials usually lead to the formation of formate as the major product; while for the
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metals, e.g. Au, with medium hydrogen evolution overpotential and weak CO interaction, CO turns to be the major product. For Pt and Fe, as they have very low hydrogen evolution overpotential and strong CO bonding ability, the major product is preferable to be H 2. An exception is Cu, which can still improve the reaction of adsorbed CO with hydrogen to form CH4 and CH3OH [42,49].
25
Noble metals, e.g., Pd, Pt, Au etc., are excellent cocatalyst to improve the reaction rate and vary the selectivity. For pristine TiO2, CO was the main product with only traceable amount of CH4. By stark contrast, after loading with Pd and Pt, CH4 became the main product instead of CO [201]. Fine structure of metal NPs also shows important effect on activity and selectivity. For example, the edge sites of Au nanowires are highly beneficial for CO generation. The
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increase of edge to corner ratio of the Au atoms on NP surface led to higher production of CO.
In general, metals loaded on semiconductor surface mainly act as electron traps to promote
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electron-hole separation, and form a high local electron density area at the metal, which is favorable for the generation of CH4 [139]. 2) Surface chemical effect
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The catalysts are obtained through various synthetic methods, e.g., hydrothermal, wet-synthesis,
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physical treatment etc., leading to different surface features of the materials. Catalysis is
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fundamentally a surface reaction, which highly relies on the surface chemistry of the catalyst.
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Therefore, surface features of the catalyst, such as functional groups, play a crucial role
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affecting the CO2 reduction from the aspects of activity and selectivity. As water is involved in nearly all heterogenous CO2 reduction photocatalytic process, surface wetting or hydrophilic-
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hydrophobic property becomes a primary factor impacting on the subsequent steps. By varying hydrophilic-hydrophobic feature of photocatalyst, some reports have shown that selective
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production of either CH4 or CH3OH could be feasible. For example, the hydroxyl groups (-OH) on Ti-beta zeolites surface can be tuned by using OH− and F− anions during hydrothermal
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synthesis to form Ti-beta (OH) and Ti-beta (F), which showed different adsorption ability to CO2 and water [202]. CO2 photoreduction under UV light over the two catalysts demonstrated quite distinct results. Hydrophilic Ti-beta (OH) demonstrated much higher reactivity than hydrophobic Ti-beta (F), whereas the selectivity of CH3OH formation from Ti-beta (F) is much higher than that from Ti-beta (OH) (41% vs. 11% in all products), indicating that increase hydrophobicity of this material favors the selective formation of CH 3OH with a reduction of 26
CH4 proportion [202]. This observation was confirmed again by introducing F into Ti/FSM-16 to increase its hydrophobicity, where the production yield of CH 3OH was increased greatly [203]. Amine groups [204] also show impact on the reaction selectivity of product formation, since they can interact with CO2 via the formation of C-N bonding, leading to the formation of to carbamate (NH2COO-) and subsequent carbonate. This process resulted in the generation of
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carboxyl radicals and then decomposed into CO on catalyst surface as the main product.
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3) Solution pH effect
CO2 solubility in water can be changed by varying pH value in solution, which also affects the concentration of CO32− and HCO3− in solution. The different chemical species generated by
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dissolved CO2 lead to different adsorption modes and reactions on catalyst surface. It is also
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found that the pH can affect the completing hydrogen evolution during CO2 reduction. Reaction
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is preferable to produce H2 at low pH [42]. An early study [205] of electrochemical CO 2
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reduction test using Cu electrode and various electrolyte solutions showed that the adsorption strength of CO on Cu is much weaker than that on Pt. Adsorbed CO is reduced to CH4, C2H4,
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EtOH and isopropanol at more negative potentials. The product types depend strongly on the
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electrolytes employed. Formation of C2H4 and alcohols is favored in solution of KCl, K2SO4, KClO4 and dilute HCO3– with pH ~6-7, whereas CH4 is the major product in relatively
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concentrated HCO3– (pH = 9.6) and phosphate solutions (pH = ~6). The product selectivity depends on protons on electrode surface, which is controlled by pH or the electrolyte, since OH− is released in the electrode reactions. However, whether this observation in
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electrochemical catalysis can be applied in heterogenous photocatalysis is still unknown. The pH of the reaction solution may play an important role in the preferred surface reaction, leading to different final products. 2.2 Photocatalytic Conversion of Organics
27
In chemical industry, the synthesis of many important organic chemicals usually requires harsh operating conditions, such as high temperature and pressure, with extreme amount of energy consumption. Heterogenous photocatalytic organic synthesis is a concept to use light as an energy source and small hydrocarbons as the raw material (reactant) to produce value-added organics, which indicates a more promising route to drive chemical reactions. Moreover, many
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chemicals synthesized upon photocatalytic process can be achieved in simple “one-pot” reactions. The following content briefly reviews the state-of-the-art progresses of the
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heterogenous photocatalytic organic conversions. Depending on the reactions, this section is
mainly divided into four parts, including the oxidation of alcohol and aromatics, carbon-carbon
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coupling, esterification and epoxidation, nitrogen containing chemical formation.
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2.2.1 Oxidation of alcohols and aromatics
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In heterogenous photocatalytic water splitting, alcohols (e.g. CH3OH) have been mainly employed
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as hole scavengers to promote the reaction, because the redox potentials of alcohols are less positive than the VB edge of the semiconductor catalysts [206]. In principle, alcohols can be
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photocatalytically oxidized to the corresponding carbonyl compounds [207–214]. However, due to
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the difficulty of the reaction control, this reaction usually produces carboxylic acids or even CO2, which are products of over-oxidizing or fully oxidizing (decomposed) processes. Therefore, the
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control of oxidizing extent and the selectivity toward desired products is the primary concern for heterogenous photocatalytic oxidation of organics. Studies have shown that the selectivity can be influenced by many parameters, such as the medium (solvent), the photocatalyst itself, the presence
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of O2, and the type (structure) of the alcohol used. Another similar photocatalytic oxidation process is the oxidation of aromatics, which leads
to the formation of many important chemicals, such as resins and pharmaceutical products, where phenol, catechol and hydroquinone have been widely used as precursors in this reaction. The heterogenous photocatalytic oxidation of aromatic compounds was firstly reported by Fujihira et 28
al. [215], where the role of various reaction parameters, such as oxygen, solution pH, and additional of ions, have been investigated in the selectivity of the hydroxylated products. Friend et al [216] studied the formation of methyl formats by oxidizing methanol on TiO2 (110) surface. They found that exposure of the TiO2 surface to O2 is important to promote the photo-oxidation reaction in order to heal surface and near-surface defects that can serve as hole
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traps. Based on measurement using mass spectrometry and scanning tunneling microscopy, two
consecutive photo-oxidation steps for this reaction were proposed. The first step is to form
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CH3O and water via thermal dissociation of the O–H bond, followed by the formation of formaldehyde through hole-mediated oxidation of CH3O. In the second step, transient HCO is
produced from formaldehyde, which can react with residual CH3O to produce methyl formate
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as the final product. CeO2, as a reducible metal oxide, can replace TiO2 as the supporting
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material of Au NPs, which may endow different catalytic properties. Kominami et al. [217]
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prepared Au/CeO2 plasmonic photocatalyst by reducing H4 AuCl4 on CeO2 surface with citric
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acid as reducing agent. Upon 530 nm-wavelength excitation, Au/CeO2 can catalyze the
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selective oxidation of aromatic alcohol to aromatic aldehydes with O2 in aqueous phase, and the reaction is almost quantitative. In addition, the catalytic activity of the catalyst mainly
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depends on the specific surface area of Au NPs loaded on CeO2, rather than the total amount of Au NPs. The sample with Au NPs of 30 nm demonstrated the most efficient catalytic
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performance. Compared with the single-step synthesis, the multi-step method allows the formation of the Au NPs with larger particle size and higher catalytic activity, and their reaction
A
rate is twice higher than those NPs obtained using the single-step synthesis method. The oxidation of alcohols over the Au/CeO2 composites proceeds with high selectivity, e.g. the photocatalytic system endows the oxidation of p-aminobenzyl alcohol to p-aminobenzaldehyde at high product yield (99 %) without exerting any side effect to amino groups (Figure 10) [218,219]. Due to the SPR effect of Au under visible light irradiation, hot electrons are injected to the CB of adjacent CeO2, resulting in the electron-deficient Au. The efficient oxidation of 29
benzyl alcohol may be attributed to smooth electron transfer from electron-rich benzyl alcohol to the electron-deficient Au. Because in this reaction O2 is involved, electrons trapped by O2 may form highly active oxygen species, such as O2•− radical and/or H2O2. In the presence of metal co-catalyst, one-electron reduction of O2 takes place more efficiently in photocatalytic oxidations than the multielectron reduction of O2. The high selectivity of benzaldehyde implies
IP T
that those active oxygen species may be exhausted in the reaction system., The Au based plasmonic catalyst can also catalyze hydrogen peroxide to oxidize aromatic alcohols to
SC R
corresponding carbonyl compounds [220] with a conversion rate close to 95%, under visible light irradiation. Li et al. [221] also presented a plasmonic Au/CeO2 hybrid nanofiber catalyst for selective oxidation of benzyl alcohol to benzaldehyde with O2 under visible light irradiation.
U
The Au loading effect on catalysis was studied and showed that an optimal level over 0.5 wt. %
N
Au-loaded CeO2 nanofibers led to the highest selectivity of 100 % benzaldehyde formation.
A
Shiraishi et al. [222] prepared Au/TiO2 plasmonic photocatalyst by loading Au NPs smaller
M
than 5 nm on the interface of anatase/rutile TiO2 mixed crystal structure (Figure 11). At room
ED
temperature, aromatic alcohol can be efficiently converted to oxo-compound with high catalytic selectivity using Au/TiO2 as the catalyst with visible light irradiation.
PT
Hollmann et al. [223] have developed a photocatalytic technique, combining
CC E
photochemical water-oxidation catalysis with peroxygenase catalysis to achieve visible-lightdriven, aerobic oxidation of hydrocarbons with high selectivity. The generation of H2O2 during photocatalytic
water oxidation
process
promoted
peroxygenase-catalysed,
selective
A
oxyfunctionalization reactions. This concept was testified for the reaction of the selective photoenzymatic hydroxylation of ethyl benzene using Au-TiO2 photocatalyst in the presence of Agrocybe aegerita (AaeUPO) under visible light irradiation (Figure 12-a). More importantly, a broad range of aliphatic and aromatic compounds have also been converted into their corresponding alcohols, verifying the versatility and robust of this technique. Immobilized Au 30
NPs containing layered titanate in interlaminar space as catalyst promoted the oxidation of benzene in the presence of water to the corresponding phenol with high product yield and selectivity (96%) under visible light irradiation [224]. Huang and co-workers [225] prepared a series of plasmonic catalyst M@TiO2 (M=Au, Pt, Ag) using a simple in-situ growth approach to load noble metals onto TiO2 (Figure 12-b). Among the three catalysts synthesized, Au/TiO2
IP T
demonstrated the highest catalytic activity for visible-light hydroxylation of benzene in the aqueous phase. The selectivity of the reaction can reach 91% with the phenol yield of 63%.
SC R
Tada et al. [226] found that the oxidation of alcohols into carbonyl compounds in aqueous phase could be effectively catalyzed by the addition of trimethyl octadecyl ammonium chloride
U
(C18TAC) in the system of Au/TiO2 under visible light irradiation (Figure 13) with high
N
selectivity (>99%). Compared with the reaction without surfactant, the addition of cationic
A
surfactant C18TAC to the photocatalytic system with an appropriate concentration leads to the
M
increase of the production yield with a factor of 3.3~5.7 due to the formation of stable micelles as nanoreactors which favor the surface reactions at the strictly confined region. As a result, for
ED
some individual alcohol reactants, the yield can be increased by 29.6 times.
PT
Very recently, Li and co-workers [227] reported an application of Bi2 WO6-x/amorphous BiOCl (p-BWO) nanosheets for photocatalytic activation of benzylic C-H bonds in toluene via
CC E
a selective photocatalytic oxidation process. The p-BWO nanosheets show distinct appearance of blue color from pristine Bi2WO6, with the irradiation of visible light and the presence of atmospheric O2, as shown in Figure 14 a and b. Structural characterization confirmed that
A
abundant W(VI)O6–x units are presented in the hybrid, which serve as the active sites to rapidly capture and consume the photogenerated electrons, thus effectively favoring the separation of electron–hole pairs. Due to this unique advantage, p-BWO nanosheets could effectively activate C(sp3)–H bonds in photocatalytic oxidation of toluene, which is a challengeable and significant reaction in chemical synthesis. This catalyst demonstrated a remarkable enhancement in 31
performance for the photocatalytic oxidation of toluene to produce benzaldehyde and benzoic acid with high selectivity. The highest conversion rate reached 2162μmol g−1h−1, which is 166 times higher than that using pristine Bi2WO6 nanosheet (Figure 14 c and d). Alloy NPs of noble metals with strong visible light absorption are capable of effectively increasing the conversion of several reactions, including Suzuki-Miyaura cross-coupling,
IP T
oxidation addition of benzyl amine, selective oxidation of aromatic alcohols, and phenol
oxidation. The reaction of oxygen oxidation of benzyl alcohol catalyzed by Au-Pd/ZrO2
SC R
[228,229] or Au-Cu/TiO2 [230] can be further improved by visible or solar irradiation. For AuPd/ZrO2 system, the distribution of Pd active sites and the heterogeneity of electrons around the
U
NP surface are determined by the molar ratio of Au-Pd, playing a critical role in catalysis.
N
Further study reveals that Au-Pd alloy can also drive the dissociation of α-H from aromatic
A
alcohols under visible light irradiation, favoring the oxidation reaction in a mild condition
M
instead of high temperature and highly concentrated oxygen (Figure 15) [229]. By introducing a small amount of metal ions (Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and
ED
Zn2+) in to CdS (see Figure 16), the mobility of photogenerated carriers at the interface between GR and CdS semiconductor can be largely enhanced [231]. This simple design can significantly
PT
improve the efficiency of selective photo-oxidation-reduction reactions under visible light,
CC E
including aerobic oxidation of alcohols and anaerobic reduction of nitro compounds. It also favors the suppression the recombination of electron-hole pairs, while retards the negative "shielding effect" caused by the high concentration of GR. Under the same reaction conditions
A
in the presence of CdS-GR (10 wt %) catalyst, the conversion of benzyl alcohol to benzaldehyde after 2 hours irradiation was approximately twice higher than that obtained using CdS with 5 wt % GR as catalyst [231,232]. A series of CdS-GR nanocomposites with different weight addition ratios of GR was also synthesized by a simple one-step solvothermal method. CdS-GR nanocomposites have been shown to be of high efficiency for selective aerobic oxidation of 32
alcohols and reduction of heavy ion Cr(VI) rather than water. In addition, the photocatalytic activity of CdS-GR nanocomposites can be effectively improved by using solvent stripped GR (SEG) instead of GO as a precursor of GR, which can effectively convert benzyl alcohol to benzaldehyde in water [231–233]. The ternary nanocomposite has been reported to be a more efficient photocatalyst compared to CdS-GR for the selective oxidation of benzyl alcohol to
IP T
benzaldehyde [234].
Oxidation of CH4 to methanol with high selectivity has recently been reported by Tang et
SC R
al. [235] using a Fe species decorated TiO2 photocatalyst at ambient conditions. The catalyst
with an optimal composition demonstrated a 15% conversion rate for CH4 with an alcohol
U
selectivity of more than 97%, where the methanol selectivity is over 90%. The production of
N
18 moles of alcohol per mole of iron active site has been achieved in 3 h. Various
A
characterization technique confirm the function of the major iron-containing species—namely,
M
FeOOH and Fe2O3, which enhance charge transfer and separation, decrease the overpotential of the reduction reaction and improve selectivity towards methanol over carbon dioxide
ED
production. The mechanism of this catalysis was proposed that upon photon absorption, electron / hole pairs are generated on TiO2 surface. As the CB potential of Fe2O3 is lower than
PT
that of TiO2, photogenerated electrons can be then transferred to the CB of Fe2O3, leading to
CC E
the reduction of H2O2 to form·-OH radicals. Simultaneously, CH4 can react with photogenerated holes at the VB of TiO2 to form methyl radicals (CH3), which then react with the -OH radicals to form methanol on Fe species.
A
To increase the sustainability and production of biorefinery, intensive effort has been
focused on the efficient utilization of the three components of lignocellulose from biomass, in which lignin is believed to be the most abundant source of renewable aromatics. However, challenge still exists in the direct transformation of native lignin from biomass. For example, conventional approaches, such as Kraft and Organosolv processes, usually involve the 33
fractionation of lignocellulose, which can cause transformation of lignin structure. In particular, the condensation to form more C-C linkages by consuming β-O-4 bonds, inducing the inactivation of the extracted lignin [236]. Very recently, a more advanced achievement of the heterogenous photocatalytic selective organic oxidation has been reported. The concept is based on the selective fractionation / oxidation of cheap biomass (macromolecules or polymer
IP T
composite) to form valuable organic products. A report from Wang et al. demonstrates a lignin-
first strategy based on solar energy-driven β-O-4 ether bond cleavage under mild conditions
SC R
using CdS quantum dots (QDs) as the photocatalyst (see Figure 17) [237]. The distinct
advantage of CdS QDs is their high selectivity in this reaction without segmenting other bonds, thus the functional groups of lignin compounds can be secured. More importantly, upon the
U
photocatalytic process using CdS QDs, native lignin compounds in birch woodmeal can be
N
directly converted into functionalized monomeric aromatics with a yield of 27 wt%, which
A
reaches 84 % of the theoretical maximum calculated from the cleavage of the β-O-4 linkages.
M
A new electron-hole coupled (EHCO) mechanism has been proposed for the precisely cleaving
ED
the C−O (β-O-4). In principle, the Cα−H bond is first fragmented upon oxidation by holes, forming Cα radical intermediate, which greatly decreases the bond dissociation energy (BDE)
PT
of the β-O-4 bond. Therefore, upon the photo-excitation of CdS QDs, β-O-4 bond breaking is promoted by the Cα radicals. The remarkably reduced BDE of β-O-4 bond in the Cα radical
CC E
intermediate is the dominating factor, leading to the high activity and selectivity of the lignin transformation. After reaction, CdS QDs can be facilely separated by forming aggregates upon
A
an addition of acetone into the liquid. 2.2.2 Carbon-Carbon bond formation (C-C coupling) Kim et al. [238] reported a hybrid structure composed of Pd NPs on exfoliated 2H-WS2 nanosheets (WS2/Pd). This catalyst demonstrated extraordinarily high photocatalytic activity in Suzuki reactions for C-C coupling under visible light (Figure 18). The photocatalytic activity 34
of the WS2/Pd showed size-dependence on Pd NPs. The increase of Pd NP size on WS2/Pd hybrid led to the enhancement of photocatalytic activity in Suzuki reactions at room temperature. Moreover, the protic organic solvents also affected the catalysis, which can activate WS2/PdN catalysts in photocatalyzed Suzuki reactions; whereas they had been considered less active compared with the polar aprotic ones in conventional Suzuki reactions.
IP T
A plausible mechanism suggested that photogenerated holes can be transferred to the protic
organic solvents, while the photogenerated electrons are transferred to Pd NPs, which favor the
SC R
efficient electron / hole separation. This charge transfer process endows the electronenrichment on Pt NPs and accelerates the oxidative addition of aryl halides reactions. Xie et al. [239] report the visible-light-driven dehydrogenative coupling of methanol into ethylene glycol,
U
using CdS NPs (TEM images in Figure 19) as the heterogenous catalyst and methanol as the
N
raw material under a mild condition. Conversion ratio of ethylene glycol over CdS nanorod
A
catalyst modified by MoS2 nano-foam reached 90% with high selectivity. Mechanism study
M
indicates that the activation of C−H bond rather than O−H bond in CH3OH by photogenerated
ED
holes on CdS through a concerted PET (CPET) process results in the generation of the hydroxymethyl (⋅CH2OH) intermediate on CdS surface, and the weak adsorption of CH3OH
PT
and ⋅CH2OH radical enables them to be readily desorbed from CdS surfaces for subsequent CC coupling to form EG. Additionally, the MoS2 nano-foam with rich edge position is highly
CC E
beneficial to the reaction efficiency. The reaction selectivity of EG has also been increased by the enrichment of mesoporous MoS2 nano-foam.
A
Wang et al. [162] reported a unique truncated tetragonal bipyramidal bismuth vanadate
(BiVO4, BVO) single crystal structure, with tuneable {010} and {110} facets, for photocatalytic C-C coupling of formaldehyde into EG under visible light. Chloride anions was used to control the growth of the single crystals as well as the ratio of exposed {010} and {110} facets. The BVO with the facet ratio of 1 : 1 allows the highest conversion yield of C2 compound. Pt@MoO and MoO 35
NPs were loaded to the two facets of the BVO, respectively, which further enhanced the conversion ratio (Figure 20). This greatly increased property is attributed to the unique architecture, where photo-charge separation can be largely accelerated by the Pt and MnOx NPs, while the HCHO coupling may be catalyzed at the MoOx shell via a redox process. 2.2.3 Esterification and epoxidation
IP T
Using alkyl alcohol as the reactant, ester has been synthesized via a heterogenous photocatalytic
approach [240]. With H14[NaP5W30O110] loaded silica as catalyst, Fatemeh and co-workers
SC R
successfully demonstrated the esterification of salicylic acid with fatty alcohol and benzyl
alcohol. When the reaction time is 3 h and the starting molar ratio of acid to alcohol is 1:5, the
U
yield of ester reaches 85%.
N
Zhu and Sarina [241] reported the photocatalyzed selective epoxidation of alkenes using
A
TiN supported Cu NPs (Cu@TiN) as the photocatalyst (Figure 21). During photocatalysis, a
M
chemisorb of reactant alkenes on Cu NPs takes place and form Cu-alkene complexes at catalyst surface, which can be activated by photon absorption. The cyclic ether solvent also plays a
ED
crucial role, which can facilitate the reaction by reacting with the adsorbed O2 on Cu NPs under irradiation to produce oxygen adatoms. The activated surface Cu-alkene complexes can then
PT
react with the adatoms to form corresponding epoxides. This photocatalyst exhibits excellent
CC E
activity and selectivity with various types of alkenes for photocatalytic epoxidation. Irradiation wavelength and intensity showed important effect on the epoxidation, indicating that photogenerated electrons of Cu NPs drive the reaction.
A
Plasmonic photocatalyst has also been applied for visible-light-driven selective
epoxidation of organics [242]. Ethylene can be selectively oxidized to ethylene oxide catalyzed by Ag cubes supported on Al2O3 with the side length of 60 nm under visible light. At 450 K, the oxidation rate under visible light irradiation is 4 times higher than that in the conventional heat-driven system. The increase of reaction rate is dependent on the temperature, which varies 36
from 8 times at low temperature to 3 times at high temperature, without affecting the selectivity in reaction [243]. Under ambient conditions with visible light, the ZnS-GR nanocomposite exhibited photocatalytic activity for aerobic selective oxidation of alcohol and epoxidation of alkenes. The GR in ZnS-GR works as an organic dye-like macromolecule "photosensitizer" for ZnS rather than an electron reservoir. Cu can also be used as a catalyst for some reactions, but
IP T
it is rarely used to catalyze epoxidation of alkene as it can be easily oxidized. Linic et al. [244] used Cu NPs with an average size of 41±9 nm attached on SiO2 (Cu/SiO2) for catalyzing the
SC R
epoxidation of propylene. Under visible light irradiation, the selectivity of the reaction is greatly
enhanced. The reason is that the localized surface SPR effect of CuNPs weakens the Cu-O bond, thus promoting the reduction of Cu2O to Cu0 and the consequent electron transport.
U
2.2.4 Carbon-Nitrogen bond transform
N
Photocatalytic N-alkylation of ammonia in alcohol to amines has been studied by Ohtani
A
et al. in an early report, using Pt-TiO2 as photocatalyst, where Pt plays an important role of the
M
selectivity of the reaction [245]. Secondary amines have also been synthesized using primary
ED
amines in alcohols aqueous solution (Figure 22) [245]. Pt decorated TiO2 and CdS catalysts showed good activity to synthesize amino acids (e.g. glycine, alanine, serine, aspartic acid, and
PT
glutamic acid) in an aqueous solution of ammonia and methane [246–248].
CC E
Recently, Stíbal et al. [249] reported the photocatalytic N-alkylation of aniline with AuTiO2 as catalyst and primary alcohols as reactant (Figure 23). The overall selectivity of the mono alkylation and two alkylation products is always higher than 70%, and depends on the
A
alkyl alcohol used. It is proposed that the reaction is triggered by the oxidation of the primary alcohol to the corresponding aldehyde through the R-O• or OH• radical intermediates by the photo-generated holes of TiO2. The photo-generated electrons transferred to the Au surface reduce the resultant protons to molecular hydrogen, which subsequently react with the aldehyde to accomplish the alkylation. TiO2/maghemite-silica nanocomposite as a photocatalyst has also 37
been reported for the selective oxidation of benzyl alcohol to benzaldehyde. When the conversion of benzyl alcohol is about 50%, the unprecedented selectivity of benzaldehyde in organic medium reaches 90% [250]. Nitroaromatic compound (e.g. 4-nitrophenol (4-NP)) can be reduce onto corresponding aminoaromatic (e.g. 4-aminophenol (4-AP)) in the presence of Au or Pt catalyst with NaBH4
IP T
as the reducing agent [251–253]. A recent study from Ma’s group [254] showed that the reaction rate of this reduction process can be further increased under photo-irradiation using a Au-TiO2
SC R
catalyst. As illustrated in Figure 24, it is proposed that under UV light, Au works as electron sink to accept the photo excited electrons from the CB of TiO 2. The electrons are then injected
U
into the LUMO of the 4-NP for reduction. Meanwhile, the high electron density on Au favors
N
the formation of hydrogen or Au-H species on Au surface for more efficient hydrogenation to
A
4-NP. On the other hand, under visible light, a hot-electron mechanism plays dominant role for
M
reaction rate enhancement. Due to the SPR effect of Au, hot electrons were injected to CB of TiO2, which can be further transferred to the LUMO of 4-NP for reduction to 4-AP.
ED
Nitrobenzene can be transferred to imines upon the photocatalytic process using TiO2 as the
PT
photocatalyst and alcohol as dispersant (Figure 25). This reaction showed high selectivity for the cases using C1–C3 alcohols [255]. It has shown that surface properties of the TiO2 exerts important
CC E
effect on selectivity [256]. Anatase structure with a much higher Lewis acidity than rutile one can favor imine formation, while Rutile exhibited a higher selectivity for the formation of aromatic amino compound. Pt/TiO2 [257] and Au/CeO2 [258,259] are able to promote photocatalytic
A
azobenzene reduction using alcohol as a sacrificial electron and proton donor. Very recently, Shiraishi et al. [260] reported that the photocatlytic hydrogenation of azobenzene to hydrazobenzene on CdS NPs under visible light irradiation reached the selectivity more than 95%. Homoallyl amines can be obtained using olefins and trisubstituted imines as reactants and CdS based materials as photocatalyst. Shiraishi et al. [261] showed the study of photocatalytic cyclisation 38
reactions, where benzimidazoles were synthesized using 1,2-diaminobenzene in the presence of Pt– TiO2 photocatalysts in alcohol solution. Using nitroaromatic compounds in alcohols as the starting materials, Hakki et al. [262] reported the photocatalytic formation of polyalkylated quinolines over TiO2 decorated with acid modified mesoporous SiO2 catalyst (Figure 26). In this reaction, nitro compound reduction and the alcohol oxidation are proceeded simultaneously upon the
IP T
photogenerated electrons and holes, respectively. An imine is then produced upon condensation of
the generated aldehyde and amino compounds. The surface attached arene–SO3H acid group
SC R
strongly affected the selectivity of the products and production yield of the quinolines.
Morpholine derivatives are important substance for pharmacological study. Plasmonic photocatalysts can be used to catalyze the oxidative condensation of aldehydes and morpholine
U
under visible light irradiation. Luque group [263] found that in the presence of H2O2 and strong
N
alkali KOH, under 532 nm laser irradiation with Au/SiO2 as photocatalyst, amides can be
A
synthesized by the reaction of morpholine with benzaldehyde. MOFs have also been used for
M
imine formation from amines. Li et al. [185] reported a visible light assisted aerobic
ED
photocatalytic oxidation of amine to imines over NH2-NIL-125(Ti), where the photo-generated Ti3+ and O2•− formed through the reaction between Ti3+ and O2 promoted this organic
PT
transformation.
Unsupported Ru NPs have also been used as photocatalyst to direct aminate octanol and
CC E
other alcohols into primary amines in the presence of ammonia (Figure 27) [264]. Octylamine was obtained at a lower reaction rate using small Ru NPs (~ 2 nm) with the selectivity of ~ 90%
A
and conversion ratio of 92%; whereas for larger Ru NPs and supported NPs, a lower selectivity was observed for the octylamine formation with a conversion decreased to 70–80%. The primary reaction of alcohol amination into octylamine showed nearly structure-insensitive reaction, as all Ru NPs presented comparable turnover frequency values. The self-coupling of octylamine was sensitive to the NP size. Amine self-coupling is inhibited upon small NPs,
39
which can be attributed to the structural and electronic effects during hydrogenation of the secondary imine. 2.2.5 Factors affecting products Although value-added organics can be “one-pot” synthesized using heterogenous
IP T
photocatalytic process from simpler staring materials, which bears the distinct advantages of mild reaction conditions, less energy consumption and environmental-friendly, the challenges
SC R
exist in promoting reaction selectivity and reaction rate. The key issue in photocatalytic oxidation reactions is to minimize the mineralisation of products and reactants to achieve good selectivity, thus a suitable catalyst as well as optimal reaction conditions need to be systematically investigated. As
U
aforementioned, there are many impressing factors, such as solvent, catalyst structure and its
N
surface functionality etc., which may present together as combined effect during reaction. We
A
here briefly summarize several possible factors affecting the reaction efficiency and selectivity.
M
1) Effect of solvent
In reaction, solvent plays the role to disperse and transport reactants to the reaction sites of the
ED
catalyst, and also may join the reaction, therefore, solvent features, such as structure and polarity,
PT
strongly influences the interaction between the surface of the photocatalyst and the chemicals. By simply varying the solvent, photocatalytic reaction rate can be tuned. A report from Brezova et al.
CC E
[265] showed that the photocatalytic reduction rates of nitroaromatic compounds in different solvents follow the order as: methanol > ethanol > iso-propanol, because the formation of αhydroxyalkyl radicals in methanol is more efficient than that in other solvents [266]. Solvents with
A
different properties, such as diffusibility and polarity possess different ability to transport all species during reaction, including reactants, highly active intermediates and products, which exert important impact on electron transfer kinetics, subsequent reaction rates and selectivity [267]. Almquist et al. [268] reported the solvent effect using the reaction of TiO2 catalyzed oxidation of cyclohexane in various solvents including acetone, isopropanol, dichloromethane, chloroform, carbon tetrachloride, 40
benzene and n-hexane. It was found that selectivity to cyclohexanol, cyclohexanone and reaction rates were influenced by the type of solvent. Non-polar solvents (e.g. hexane and benzene) led to low selectivity with cyclohexanol being completely oxidized to water and CO2, due to their strong adsorption on catalyst surface. In polar solvents, adsorption of cyclohexanol to the TiO2 NPs is weaker as the competition for adsorption sites with the solvent, and thus the selectivity is higher.
IP T
Solvent induced competitive adsorption, to an extent, determines the overall reaction rate and
selectivity to cyclohexanone and cyclohexanol. Dichloromethane allowed the highest reaction
SC R
rate; by contrast, chloroform and isopropanol dramatically inhibited the desired reactions. The
optimal solvent for this reaction with high selectivity is the one that can minimize product adsorption on catalyst and improve reactant adsorption on catalyst surface. Colmenares et al.
U
[250] also reported the similar phenomenon, where photocatalytic oxidation of benzyl alcohol to
N
benzylaldehyde in acetonitrile demonstrated better selectivity than that in aqueous medium. Soana
A
et al. [269] reported the effect of an organic solvent to slow down the reaction and improve selectivity.
M
Water is a good “green” solvent for heterogenous photocatalytic reactions, it is of high polarity
ED
and can easily interact with the surface of the photocatalysts to react with photo-generated holes to form strong oxidizing radical of –OH•, thus resulting in a difficult control to the oxidation extent and
PT
hence inducing a poor selectivity. Therefore, polar organic solvents (e.g. the alcohols introduced above, dimethyl carbonate [270] and acetonitrile) or their mixtures are most often employed as
CC E
dispersant for photocatalytic organic synthesis. For instance, acetonitrile has been used as solvent in many photocatalytic organic syntheses, including oxidation of liquid cyclohexane [271], benzyl
A
alcohol [250], naphthalene [272], primary aliphatic amines [273], and epoxidation of propylene with high selectivity [244], as it is a stable solvent which usually cannot be oxidized by the photogenerated holes. The pH value of the solution also affects the dissociation of the reactant as well as their adsorption on catalyst surface. For example, Zhu et al. [274] reported an efficient photocatalysis to selectively oxidize 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under mild conditions using O2 in air as the oxidant and g-C3N4 supported cobalt 41
thioporphyrazine (CoPz) as photocatalyst. This reaction showed pH dependence of the solution, where the FDCA can be achieved with a yield of 96.1% in an aqueous solution at pH = 9.18, while at lower pH (4.01), the main product is 2,5-diformylfuran. 2) Effect of catalyst surface feature In heterogenous photocatalytic organic synthesis, reaction is proceeded following two major
IP T
steps [12,13,24]. The 1st step is the adsorption/dissociate process, where reactants adsorbed on catalyst surface; the 2nd step is the desorption process, where products leave the catalyst surface.
SC R
Therefore, strong adsorption between organics and catalysts (mainly semiconductors) usually
induce high reaction rate, however it may lead to the mineralisation with CO2 and water as the final products. A rational design to the catalyst surface, such as specific functional groups, can
U
result in a suitable and preferential adsorption, and thus increases the selectivity of the reaction.
N
Hakki et al. [262] reported that the formation of polyalkylated quinolines via a cyclization using
A
nitroaromatic compounds as reactants reaches a selectivity up to 53% catalyzed by arene–SO3H acid
M
group functionalized mesoporous SiO2-TiO2 hybrid, whereas this value is only 6% for pure TiO2,
ED
and 18% for SiO2-TiO2. Notestein et al. [275] designed a novel catalyst with nanocavities constructed by depositing a shape-selective sieving layer of Al2O3 on an oxide (TiO2) catalyst
PT
surface, which bears high selectivity for both reduction and oxidation reactions as shown in Figure 28. Nitrobenzene and benzyl alcohol can be photocatalytically reduced and oxidized into
CC E
their corresponding products with selectivity up to 90%, respectively, while no reaction was taken place for the ortho methylated derivatives. The excellent selectivity is resulted from the
A
size-sieving effect induced adsorption alignment of reactants to the catalyst surface. Ohtani et al. [276] found the selectivity of photocatalytic synthesis of pyridine 3-carboxylic acid (PCA) from diamines using CdS is higher than that using TiO2 as photocatalyst. High purity racemic PCA product was obtained in the presence of CdS, whereas for the case using TiO2, PCA was produced with an excess of the L-isomer as by-product. The selectivity difference was ascribed to the
42
adsorption difference of the amino group in l-Lys on catalyst surface, which underwent distinct oxidation reactions [277]. 3) Effect of incident light wavelength The efficiency of charge carrier’s generation on photocatalyst is of high importance for photocatalysis, which depends on the energy levels of the semiconductor and the energy of the
IP T
incident light. By varying the wavelength of the incident light, the process for photogeneration of electron-hole pairs may be tuned to favor the selectivity and reaction rate for the organic
SC R
transformations.
For example, as shown in Figure 29, under visible light irradiation (λ> 450 nm), Pt NP decorated P25 (Pt/P25 catalyst) favored selective aerobic oxidation of aniline to form nitrosobenzene as the
U
main product (selectivity: 90%) [278]. By contrast, under UV irradiation azobenzene turned be the
N
major product. It is proposed that upon visible light absorption, an inter-band electronic excitation
A
of Pt atoms takes place, which promotes the electrons to be transferred to the CB of TiO2, followed
M
by the formation of O2˙− radicals on Pt surface. The O2˙− radicals are highly active to attract the H
ED
atoms from aniline upon its dissociation on Pt NPs, producing hydroperoxide species. The following reaction between anilino anion and hydroperoxide species results in the formation of nitrosobenzene.
PT
Pt/TiO2 photocatalyst has also been used to catalyze aromatic ring hydroxylation of benzene derivatives when illuminated with light of appropriate wavelength, in the presence of water as
CC E
oxidant [279]. It was observed that under irradiation with 365 ± 20 nm light, the selectivity of phenols, produced via photocatalytic hydroxylation of toluene or p-xylene was low; however, the yield and
A
selectivity was significantly increased under 405 ± 20 nm light irradiation (Figure 30). This wavelength dependant selectivity is attributed to that upon incident light at ~ 405 nm photogenerated charge carriers mainly form at the surface of the catalyst, which are more efficient to participate the hydroxylation reaction than those generated from the deeper area of the catalyst excited under ~365 nm light. The lower selectivity at 365 nm was resulted from the retardant of charge separation due
43
to the electron transfer from toluene to TiO2 upon the absorption of toluene at ~ 360 nm, leading to the generation of benzyl radicals to form dibenzyl. 3. Photoresponsive NP Initiated Radical Polymerization Apart from the photocatalytic synthesis of small organic molecules, we here introduce the advances of heterogenous photocatalytic synthesis of polymers or macromolecules from their
IP T
monomers, to demonstrate the fundamental achievements including the reaction mechanisms
and technical advantages. Compared with CO2 reduction and organic transform introduced
SC R
above, this domain is relatively less developed, because it needs more background from polymer chemistry and specific characterization technique of macromolecules, including
U
structure and mechanics analysis, differing from those for small molecules. We choose the free
N
radical polymerization system, as it is the most classic and major topic in polymer research.
A
3.1 Free Radical Polymerization
M
3.1.1 Simple semiconductor system
The use of semiconductors to photo-initiate polymerizations were first attempted by Markham
ED
and Oster et al. [280,281]. Both reported that ZnO NPs can initiate the free radical
PT
polymerization of methyl methacrylate (MMA) under UV ( = 365 nm) irradiation. They also demonstrated that the presence of O2 adsorbed on ZnO surface is crucial for the reaction, as no
CC E
catalytic activity could be observed when using ZnO NPs undergone a thermal treatment, where the surface adsorbed O2 was removed upon heating. However, excess O2 in solution can inhibit the polymerization, as expected for any radical polymerization. Solvent effect study showed
A
that solvents of higher dielectric constants with good proton-donating or hydrogen-bonding characteristics favor the initiation reaction via the intervention of photo-excited surface O2. The anionic nature of the excited surface oxygen was evidenced by the fact that the polymerization can proceed readily for electrophilic monomers such as MMA and acrylonitrile, but not for styrene [280]. It was thus proposed that photo-activated oxygen was formed on the surface of 44
ZnO during irradiation and converted into radical anion (such as O 2 •-) which could then initiate the polymerization [280,281]. The photo-generation of active species on the semiconductor surface and the following initiating step of photo-radical polymerization were further investigated using quantum sized TiO2, ZnO and CdS by Hoffman and co-workers [256,282]. The effect of solvent, monomer
IP T
concentration, NP concentration, light intensity and NP size on polymerization rate were examined. The quantum sized semiconductors exhibited significantly higher quantum yields
SC R
than their bulk counterparts. Figure 31-a presents the proposed reaction mechanism consisting of the surface reduction of the monomer by the photoexcited electrons present at CB, leading to a radical anion which can initiate the free radical propagation. An alkyl alcohol solvent (such
U
as isopropanol) must also be present in the system. It acts as hole scavenger in order to suppress
N
charge recombination, and the resulting oxidation product can also initiate the polymerization,
A
either directly or via monomer oxidation. The implication of a radical anion as pivotal initiating
M
species was confirmed by the inhibition of the reaction by O2 or acetonitrile, both of which are
ED
not only electron scavengers, but also radical anion scavengers. For the case of the MMA polymerization triggered by ZnO NPs, Hoffman noticed that the reduction potential (E Ared) of
PT
MMA (-1.1 V vs NHE) is more negative than the energy of CB (E CB) of ZnO (-0.2 V vs NHE). Thus, MMA should not be reduced by photo-excited electrons. It was proposed that stored
CC E
electrons with more negative redox potential (< -2.0 V), and thus potentially able to reduce MMA, may be localized at surface-trap sites or trapped under the form of Zn+ ion at the NP
A
surface [283]. Later investigations using annealed ZnO NPs in an unstirred solution pointed toward reaction details which differ from the polymerization in stirred reaction medium with non-annealed ZnO NPs [284]. In the former case, the polymerization reaction takes place preferentially at the NP surface, leading to a polymer-NP composite. Remarkably, unlike PMMA prepared by a conventional thermal radical process, the resulting PMMA product is predominantly syndiotactic with significant crystallinity [284]. 45
A disadvantage of the aforementioned ZnO and CdS NPs is their limited sensitivity to visible light [255,256,282,284,286–289]. The spectral sensitivity range was considerably expanded by using Fe2O3 NPs as photoinitiator to polymerize butyl methacrylate (BMA) in isopropanol solution under visible light [285,290]. As illustrated in Figure 31-b, since the ECB ≈ -0.45 V (vs NHE) of Fe2O3 is less negative than the EAred (-1.1 V vs NHE) of BMA monomer,
IP T
the direct transfer of the photoexcited electron from the CB to the monomer is not feasible.
However, isopropanol can be oxidized by the holes in the VB, leading to the formation of
SC R
isopropoxy radicals, as the redox potentials of 2-propanol (EDox = -0.244 V vs NHE) is less positive than the energy of VB (E VB) of Fe2O3 [285]. Moreover, the protons formed during the oxidation of isopropanol may accept the excited electrons from the Fe2O3 CB to form active
U
hydrogen species (noted, for the sake of simplicity as H▪) which can further react with BMA or
N
alcohol molecules to generate radical initiating species. Thus, although the direct electron
A
transfer from the CB of the photoinitiator to the monomer molecule is thermodynamically
M
prohibited, the electron and hole generated upon the absorption of light may nevertheless lead
ED
to formation of monomer radicals capable of initiating a chain reaction [285]. The photopolymerization of MMA using CdS was reported by Popovic et al. who proposed
PT
a different mechanism based on the analysis of the thermal stability of the resulting PMMA [255,286]. Differential thermogravimetry (DTG) results show that the thermal stability of
CC E
PMMA synthesized with CdS photoinitiator was greater than PMMA thermally prepared with AIBN, but was comparable to PMMA prepared by anionic polymerization. The DTG curves of
A
the CdS initiated PMMA (Figure 32-a left) was composed of four peaks, three of which correspond to radical PMMA prepared with AIBN (Figure 32-a right). The additional peak of the photopolymerized PMMA suggested a CH2CH2 segment in position relative to a vinyl end-group, which is the signature of the initiation process [285]. Therefore, a plausible mechanism (Figure 32-b) was that the positive hole oxidizes a MMA molecule, leading to the formation of a proton and a very stable acrylic radical stabilized by -CO2Me. Addition of a 46
MMA unit on this stable radical leads to the formation of a propagating chain containing the CH2CH2 moiety in position of an unsaturated chain-end. It was suggested that these unsaturated chain-ends are thermally more stable than the conventional unsaturated chain ends arising from radical disproportionation, thus resulting in a photopolymerized PMMA with improved thermal stability [285].
in a non-degassed solution [291]. Using
1
IP T
The MMA photo polymerization reaction was performed using TiO2 NPs as photoinitiator
H Nuclear Magnetic Resonance (1H-NMR)
SC R
spectroscopy, the polymer was found to contain 6.9 % of isotactic, 38.1% of heterotactic and
54.9% syndiotactic triads. Such a microstructure is consistent with a radical PMMA. Interestingly, DTG analysis and theoretical calculations indicate that a fraction of the PMMA
N
U
chains possess the characteristic CH2CH2 moiety to an unsaturated chain-end (red area in Figure 32-c), implying an initiation mechanism similar to the CdS-MMA system (Figure 32-c,
M
A
(I)). In this study, due to the presence of dissolved O2 in the non-degassed liquid medium, O2•species can be formed by electron transfer from the CB of photoexcited TiO2 to O2, which then
ED
react with holes to generate the highly reactive OH • radical. These hydroxyl radical are known to initiate MMA radical polymerization (Figure 32-c, (II)), as observed in the case of a
PT
conventional MMA polymerization initiated by a redox Fe2+/H2O2 system. The generation of
CC E
OH• radicals was detected experimentally by Electron Spin Resonance (ESR) spectroscopy using dimethyl pyridine N-oxide (DMPO) as spin trap, thus confirming the proposed mechanism where both holes and OH • radicals work to generate initiating species [291].
A
The improvement of the semiconductor quantum efficiency is essential to enhance the
reaction efficiency [294,295]. After dye sensitization, the oxidation power is measured by the HOMO of the dye, which is usually less powerful as an oxidant than the electron hole [295]. Dye sensitized semiconductors, i.e., CdS and ZnO NPs, demonstrate significantly enhanced activity as initiators in photopolymerization [288,292]. Xanthene dyes in combination with ZnO 47
NPs are able to trigger the photopolymerization of BMA under visible light [288]. Eosin was used to sensitize CdS NPs for the photopolymerization of MMA [292], leading to a higher polymer yield in comparison to the system initiated with CdS NPs alone, or with CdS in combintation with triethylamine, NEt 3. Analysis by ESR spectroscopy confirmed that the CdS + eosin system generated more free radicals than the two other systems (Figure 33-b). The dye
IP T
sensitized CdS photoinitiator also showed good photostability for a long time because the dye
molecules were not photodegraded. It is believed that (Figure 33-a) upon light absorption, an
SC R
electron transfer occurs between the photoexcited dye (D*) and the CB of the semiconductor. The electrons in the CB band can then be transferred to the monomer molecules, leading to the
formation of an organic radical anion which can initiate polymerization [292]. Carbon doped
U
TiO2 also demonstrated visible / solar light activity to initiate the polymerization of MMA on
N
NP surface to form PMMA-grafted TiO2 nanostructure [296]. Compared with non-surface
A
confined PMMA, the photopolymerized PMMA confined at the surface the TiO 2 particle
M
possesses a high thermal stability as shown by TGA analysis. As shown by XRD, it is also
ED
crystalline, and its unit cell is unlike the one of syndiotactic PMMA which is obtained by low temperature radical polymerization. Hybrid polymer TiO2 nanocomposites obtained by
PT
photochemical grafting also exhibit interesting properties. For example, the in-situ polymerization of amine-containing 2-(tert-butylamino)ethyl methacrylate and ethylene glycol
CC E
dimethacrylate initiated by TiO2 NPs led to the formation of a core@shell TiO2@polymer nanocomposite which exhibits high antibacterial activity. This antibacterial activity is ascribed
A
to the combined action of the photocatalytic degradation by TiO 2 NPs, the biocidal polymeric amine containing polymer and the antifouling characteristic of the ethylene glycol units (Figure 33 c-f) [293]. Ravoo and coworkers developed a TiO2 NP photoinitiating system to carry out step-growth polymerization, which can be potentially be used for microcontact printing in a simple, rapid and economic way [297]. This technique took advantage of photoinduced electron / hole charge 48
carriers from semiconductors to induce reduction / oxidation of ethanol amine to initiate its step-growth polymerization. As illustrated in Figure 34, the photogenerated hole species of TiO2 oxidized the ethanol amine monomer (or the alcohol groups on the substrate surface) to the corresponding aldehyde, which reacts with another ethanol amine molecule to generate an imine. This imine was finally reduced by the photoexcited electrons of CB to form a secondary
IP T
amine, leading to the step growth polymerization of ethanol amine. A similar step-growth polymerization was also reported in the synthesis of polypyrrole using TiO2-based
SC R
nanostructures [298].
To increase the initiation efficiency, co-initiators, such as amines can be introduced to the photopolymerization system to promote the generation of free radicals as reported by Yagci et
U
al. when using ZnO NPs [5,299]. In degassed water, in the absence of co-initiators, the
N
photopolymerization of acrylamide can be initiated by ZnO alone due to the formation of OH •
A
radicals from the reaction of H2O / O2 with charge carriers (Figure 35-a). However, this reaction
M
is completely inhibited in organic medium. Co-initiators such as tertiary amines (TEA) and
ED
iodonium salt (Ph2I+PF6-) can be added to the system to trigger the photopolymerization of MMA in the absence of water. It is proposed that the lone pair on the N atom of TEA reacts
PT
with the photogenerated hole to form a radical cation, which can initiate the polymerization (Figure 35-b). In the case of the iodonium salt, a different route is proposed, consisting in the
CC E
direct reduction of the iodonium salt by the photogenerated electrons, as shown in Figure 35-c. Carboxylic acids can also work as co-initiator in photopolymerization [300,301]. Fe3O4 NPs
A
stabilized by lauric acid (LA) ligands (Figure 36-a) are capable of initiating the polymerization of MMA in the absence of any co-initiators, as the interaction of the photogenerated holes with the carboxylic acid groups leads to the decarboxylation of LA and the formation of free radicals, which can then initiate the polymerization (Figure 36-b). However, in the presence of TEA, αamino radicals can form and initiate the polymerization (Figure 36-c). A similar
49
decarboxylation process has also been reported during the photoinduced polymerization initiated by TiO2 NPs in the presence of butyric acid [302]. It is well-known that halide perovskites in optoelectronic applications can be in part attributed to their high absorption and long carrier lifetime, leading to a priming material for the application of photocatalysis and photovoltaics. Very recently, Tüysüz et al. [303] reported
IP T
a visible light driven in-situ photosynthesis of poly(3,4-ethylenedioxythiophene), PEDOT, over cesium lead iodide (CsPbI3) Perovskite QDs (Figure 37). Higher concentration of the CsPbI3
SC R
QDs increased polymerization rate, and after synthesis CsPbI3 QDs were encapsulated and stabilized by PEDOT to form a composite. During catalysis, 1,4-benzoquinone was used as the electron acceptor to promote polymerization, and to secure the desired cubic crystal structure
U
of CsPbI3 perovskite from oxidation. This novel approach enables a single-step synthesis of
A
3.1.2 Heteronanostructure or hybrid system
N
CsPbI3/PEDOT composite, which could be promising for optoelectronic applications.
M
Semiconductor-metal composite heteronanostructures have been widely used as
ED
photocatalyst for photosynthesis. Recently, ZnO@Ag composite nanostructures have been applied as photo-initiator to triger the free radical photopolymerization [302]. As shown in
PT
Fugure 38 a - c, ZnO@Ag heteronanostructure was prepared by depositing Ag NPs on ZnO nanoflowers using a redox active poly(ionic liquid), poly(1-vinyl-3-butylimidazolium
CC E
ascorbate) (P[VBuIm][As]). Under UV light irradiation, this material demonstrated good activity and stability (Figure 38 d and e) to initiate the polymerization of MMA and N-
A
isopropylacrylamide (NIPAM) in various solvents, such as water and dichloromethane (DCM), indicating a high versatility of this initiator. In comparison with ZnO nanoflower alone, the ZnO@Ag NPs showed much higher activity in photopolymerization, which can be ascribed to that the Ag NP on ZnO surface works as electron trap promoting the separation of photocharges as well as suppressing the recombination of electron-hole pairs at surface. Moreover, the authors proposed that the adsorbed H2O molecules on NP surface, due to the presence of highly 50
hygroscopic P[VBuIm][As], are critical for the generation of OH▪ radical species to initiate the polymerization, since no polymerization occurred if the reaction system was completely free from traces of moisture [303,304]. Besides inorganic semiconductor NPs, g-C3N4 can also be used as photoinitiators, due to their unique properties combining the characteristics of a middle-band semiconductor and
IP T
conductive graphite carbon with high surface area [305]. Indeed, g-C3N4 was found to be an active visible light catalyst, useful in water splitting, photdegradation of pollutants,
SC R
Friedel−Crafts reactions, reduction of CO2 and selective oxidation [305]. Yagci and co-workers reported that mesoporous g-C3N4 in the presence of alkyl amines as co-initiators can photoinitiate the free radical polymerization of MMA under visible light [305]. The initiation
A
N
being photooxidized in an initiating radical cation [299].
U
mechanism was presumed to be similar to the ZnO-TEA-MMA reaction system, with the amine
M
3.2 Controlled/Living Radical Polymerization
Despite the popularity and economic feasibility of free radical polymerization, the major
ED
limitation of this synthetic technique is the poor control over the synthetic process that usually
PT
generates wide polydispersity of molecular weight (Mw) and difficulty to achieve well-defined polymers with controlled composition, chain architecture, and site-specific functionality
CC E
[48,306]. Controlled / living radical polymerization (CRP) has revolutionized and revitalized the field of synthetic polymer chemistry over the last twenty years and it is now possible to prepare a wide variety of previously inaccessible macromolecules under relatively mild
A
conditions [48,306]. CRP realized the well-defined microstructure design of the polymers in a controlled fashion to synthesize bioconjugates, organic/inorganic composites, and surfacetethered copolymers with a narrow Mw polydispersity [48,306–308]. Several methods including transition-metal-catalyzed atom transfer radical polymerization (ATRP), reversible additionfragmentation chain transfer (RAFT) polymerization, single-electron-transfer living free radical 51
polymerization (SET-LRP) as well as nitroxide mediated polymerization (NMP) are increasingly well-developed CRP processes for the construction of macromolecular architectures [309]. Very recently photochemistry with CRP techniques has revolutionized for the synthesis of well-defined macromolecules [306]. In the following content, the ATRP and RAFT photopolymerizations upon photosensitive NP initiating system are highlighted.
IP T
In conventional ATPR synthesis, generally a transition metal complex catalyst, such as
Cu(I)-ligand complex, is necessary to establish the control over the polymerization
SC R
[136,306,311]. Alkyl halide compounds were used as initiators to react with transition metal catalyst to generate active initiating species in a reversible manner. In the example using cooper catalyst, the polymer growth highly depends on the redox cycle of Cu(I) / Cu(II), acting as a
U
reversible activation / deactivation process for initiation. An optimal control to the Cu(I) / Cu(II)
N
ratio is crucial to maximize the reaction rate while extending the lifetime of propagation chains
A
[299,310,312]. Liu et al. report the ATRP synthesis of several polymer brushes using TiO 2 as
M
the photo-reductant and Cu(II) complex as the catalyst to trigger the polymerization under UV
ED
irradiation via a one-electron transfer process [309]. Figure 39-a displays the mechanism of the UV-induced ATRP, where the air-stable Cu(II)-ligand was in situ reduced to active Cu(I)-
PT
ligand via accepting the photogenerated electrons from the CB of TiO2, which then reacted with the alkyl halide to produce alkyl radicals for initiating the polymerization; the Cu(I) can be
CC E
facilely recovered to Cu(II) by oxidation reaction with O 2 in air [309]. In this study, it is found that decreasing the Cu catalyst amount to ppm level did not retard the reaction significantly.
A
The living nature of this UV-ATRP synthetic process was verified by its ability to form copolymer brushes (Figure 39-b, c). Under irradiation, a new chain segment of poly(sulfopropyl methacrylate) (PSPMA) can be grafted onto the chain-end of the pre-synthesized poly(dimethylamino)ethyl methacrylate (PDEAEMA) to form a block polymer brushes (Figure 39-c) [309]. The break-off and re-start of the ATRP polymerization can be simply programmed by interval light on-off (Figure 36-d) [310]. Moreover, under irradiation, if successive monomer 52
was introduced to the reaction, the polymer chain can grew continuously (Figure 39-e). Both observations confirmed the living nature of the RAFT process with NP photoinitiator. The M w of the synthesized PMMA has a small polydispersity and the experimental polymer conversion ratio showed good correlation with theoretical calculation. Polymer brushes were also synthesized through the visible-light-driven ATRP synthesis in the presence of dye-sensitized
IP T
TiO2 NPs and TiO2-rGO) nanocomposite [313,314]. The light induced ATRP allows the
application potential of micro-contact printing, where multi-functional groups with complex
SC R
architectures can be built / arranged orderly along the brush chain. On account of the same oneelectron transfer principle, ZnO, Fe-doped ZnO NPs, g-C3N4 NPs were also used as photo-
U
initiators in ATRP synthesis [299,310,311].
N
Recent research efforts have also been concentrated on developing new catalytic system
A
which can work efficiently under milder and environmentally benign conditions. For example,
M
instead of using toxic Cu, more biologically tolerable metal complexes including Ru and Fe at ppm level have been reported in ATRP synthesis [299,310,311], offering a “greener” reaction
ED
environment [5]. In addition, metal-free photoinitiated ATRP using semiconductors has recently reported by Liu and co-workers, where Nb(OH)5 NPs with benzyl alcoholate ligand
PT
(NbBA) were used as the visible light active initiator (Figure 40-a) [315]. In the presence of
CC E
NbBA and Ethyl 2-bromoisobutyrate (EBiB) as catalyst, the ATRP synthesis of poly(isopropylacrylamide) (PNIPAAm) was initiated under visible light. The proposed mechanism was illustrated in Figure 40-b, upon the photoexcitation, electron transfer occurs
A
via ligand-to-metal charge transfer process from the surface complex ligand to the metal atom of the NP, leading to the formation of radical cations of the ligand followed by the generation of a –O-CH•-Ph radical after losing the α-hydrogen. The resulted alkoxy substituted radical is highly active to react with the EBiB catalyst to yield the alkyl radical to initiate polymerization. As presented in Figure 40-a, the regeneration of NbBA, homolysis of the C-Br bond, chain 53
propagation can take place repeatedly in the cyclical process. Without using EBiB catalyst, this reaction undergoes a solely free radical polymerization process with poor control over M n and polydispersity. The NPs can be easily separated by centrifuge and reused for successive reactions. Another advantage is that this metal free reaction system enables the producing of clean products without additional efforts to remove the ionic metal residues [315].
IP T
Compared with the ATRP synthesis, RAFT polymerization renders a metal ion-free environment, which is promising for biological relevant applications. The NbBA NPs were also
SC R
successfully applied in visible light-driven RAFT polymerization, in which PNIPAAm with controlled Mn and narrow polydispersity index was obtained if the feeding ratio of NP to RAFT
agent was controlled properly [316]. Like the mechanism proposed in the above ATRP case, in
U
RAFT synthesis, the photoinduced –O–CH•–Ph radical can attack the C=S bond of the RAFT
N
agent to generate the highly reactive intermediate, initiating the RAFT polymerization (Figure
A
40-c). Afterward, C=S bond of the RAFT agent can be reformed with a radical fragment by the
M
decomposition of the radical intermediate. The raidical fragment can initiate the chain
ED
propagation and subsequently react with the NbBA-RAFT to produce a highly active polymeric intermediate again (f), which could decompose into the polymer with RAFT active center (g)
PT
and NbBA NP for the following repeating reactions. Like the ATRP process, the wellcontrolled manner of this photo-induced ATRP was evidenced by light on-off test (Figure 40-
CC E
d). The living nature was confirmed by a continuous growth of the polymer chain upon the introduction of additional monomers under irradiation (Figure 40-e) [316].
A
3.3 Free Radical Photopolymerization Induced by Other NPs Apart from the semiconductors, other photo-responsive nanostructures have been developed to assist the radical polymerization. Lanthanide-doped upconverting NPs (UCNPs) have revealed profound potential as fluorescent probes in bio-applications, such as bio-imaging and bioassays [317]. In particular, UCNPs is able to transfer low-energy near-infrared (NIR) incident photons 54
to high-energy UV or visible emission based on the sequential absorption of two or more photons. NIR light serving as the excitation source offers a distinct advantage for bio-system, as it enables higher penetration depth in tissues without photo-damage to living cells. Haupt et al. [318] reported generic strategy to construct a cross-linked polymer coating with plenty of optional functional groups on UCNPs. As illustrated in Figure 41, upon the NIR (λ = 980 nm)
IP T
irradiation, the UV emission was generated by the UPCNs, which was then captured by the dye molecules to generate active species [318]. Co-initiators were activated by the excited dye to
SC R
generate free radicals initiating the polymerization. Since the emission from the UCNPs is weak as compared to the direct light, the polymerization is proceeded only at the confined, close proximity of the UCNP surface, and thus resulting in the formation of a UNCP@polymer
U
core@shell structure. By simply varying the monomers, polymer coating with various
N
properties can be programmed facilely, which can be hydrophilic or hydrophobic, bio-
A
compatible or sensitive, chemically reactive or inert. Multilayer polymeric coating was also
M
achievable by sequentially introducing different monomers with variety of functionalities [318].
ED
SiO2 NPs are considered to be non-toxic, biocompatible materials with unique surface features, such as the rich of –OH groups, high porosity as well as large surface area, which
PT
render them to be a versatile platform for surface functionalization to meet various applications. Ruthenium complex (RuL2+) was immobilized in a Nafion (Nf) coating wrapped on SiO2 NP
CC E
to form a visible-light-active hybrid catalyst (SiO2/Nf/RuL) for radical polymerization (Figure 42-a) [319]. Due to the presence of a great number of negatively charged sulfonate groups in
A
Nafion, RuL2+ can be bound to surface of the SiO2@Nf structure, attributed to a strong electrostatic attraction between RuL2+ and Nifion (Figure 42-b). In the presence of the SiO2/Nf/RuL, free radical polymerization of MMA can be performed at ambient temperature under visible light irradiation using EBiB as initiator. As shown in Figure 42-c, the RuIIL confined in the Nifion coating was excited to Ru*L upon photon absorption, which is reduced by an electron donor, iso–dipropylethylamine (iPr2NEt), via electron transfer process, to form 55
a highly active single electron reductant RuIL. The interaction of RuIL and EBiB initiator produced a carbon-centered radical for polymerization and reduced RuIIL back to RuIL again in the photoredox cycle [319]. Schlögl and co-workers developed photoactive SiO2 corephotocleavable shell NPs by immobilizing bis(acyl)phosphane oxide and aromatic αhydroxyketone derivatives on SiO2 NP surface using silanization method, which can efficiently
IP T
undergo Norrish type I photofragmentation reactions [320]. They studied the photo-activity of these photoactive core-shell structures as photo-initiators for free radical polymerization of
SC R
thiol-ene. Results revealed that the photoactivity of the NPs is not only influenced by the type
of initiating species of the organic shell but also by the grafting method used for NP synthesis, since the latter can control the size, dispersion and surface functionalization of the NPs [320].
U
Metal NP-polymer composites have attracted increasing research interest for the
N
development of functional materials with unique electrical, optical or mechanical properties in
A
the application of optics, bio-imaging and sensors [321,322]. In particular, Au NPs have been
M
an intense research topic due to their fascinating properties and catalytic activity in various
ED
reactions [323–325]. Neckers and co-workers studied the polymerization of acrylates using 5Mercapto-2,2-bithiophene (BTS) capped Au NPs under UV and visible light irradiation, where
PT
the function of the Au was highly dependent on the excitation light [326,327]. Upon exposure to the UV irradiation (λ = 350 nm, corresponding to BTS absorption), the BTS ligands on Au
CC E
surface were excited to generate highly active radical cations at the thiophene rings of Au NPBTS+• via electron transfer to the conductive Au NP, which consequently became the unstable
A
acryl radical through deprotonation to initiate the polymerization (Figure 43-a). As the polymerization was only taken place in a close vicinity to the NP surface, which strictly localized reaction largely reduced the rate of product diffusion. On the other side, visible light irradiation (λ > 450 nm, Figure 43-b) led to different photo-radical generation mechanism due to the SPR effect of Au [326]. Au NPs can act as electron donors upon excitation at the SPR band to generate active surface species, although BTS is not light-responsive at visible region. 56
It is suggested that on account to the excitation of Au NP SPR in the presence of an oxidizing chemical of [4-[(octyloxy)phenyl] phenyl] iodonium hexafluoroantimonate (OPPI) as coinitiator, electron was transferred from the Au NP to OPPI inducing the decomposition of OPPI to form phenyl radicals for polymerization. This proposal was evidenced by the fact that without the presence of monomers, intraparticle coupling took place though BTS ligand connection to
IP T
form NP aggregates via intramolecular electron transfer and subsequent charge recombination.
Baymberg et al. reported SPR-induced radical polymerization via plasmonic “hot”
SC R
electrons using Au NPs [328]. As illustrated in Figure 43-c, Au NPs were deposited on a Au
“mirror” surface with tunable gap, where strong electric fields were formed due to the coupling of Au substrate and SPR. Under visible light (laser) illumination, N-isopropylacrylamide
U
(NIPAM) polymer growth was triggered around the junction of the Au NP and substrate,
N
without using any additional initiator. It is proposed that upon the absorption of visible photons,
A
due to the strong localized SPR electric fields, hot-electron transfer takes place from Au NP
M
surface to monomers to form initiating free radicals for polymer growth, demonstrating the
ED
radical polymerization initiated by light-directed programmable plasmons [328,329].
PT
3.4 Reaction Kinetics in Polymerization According to classic kinetic theory of a conventional free radical polymerization reaction with
CC E
a given monomer concentration, the overall rate of polymerization, Rp, is given by the following equation [47]:
𝑑 [𝑀 ] 1/2 = 𝑅𝑝 = 𝑘𝑝 𝑘𝑡 [𝑀]𝑅𝑖𝑛 1/2 𝑑𝑡
A
−
where Rin is the initiation rate, [M] is the monomer concentration; kp and kt are the rate constants of propagation and termination, respectively, which can be referred from literature values. Clearly, in a given monomer system, Rp is proportional to the square root of the initiation rate, indicating that the reaction ratio is mainly dominated by the initiation step. In the presence of a
57
photoinitiator, the initiation process is the only photochemical step. The chain initiation rate can thus be determined from the following expression [256,282]: 𝑅𝑖𝑛 = Q 𝑖 𝐼𝑎 = Q 𝑖 𝐼0 (1 − 10−𝐴 ) where Qi is the photoinitiation quantum yield, A presents the absorbance of the NPs in solution, I0 and Ia are the incident and the absorbed light intensity. Once the monomer radical is formed,
IP T
subsequent chain propagation will comply with the classic free radical chain growth mechanism. Hence, the overall rate of a free radical polymerization under the steady state using NP
SC R
photoinitiator can be expressed as follows [256,282]:
1/2
1/2 𝑅𝑝 = 𝑘𝑝 𝑘𝑡 [𝑀](Q 𝑖 𝐼0 (1 − 10−𝐴 ))
U
Therefore, with regard to the materials, quantum efficiency is a crucial factor significantly
N
determining the rate of photo-polymerization. NP photoinitiators with high quantum efficiency
A
and good charge separation feature are of large benefit to the photopolymerization. The kinetic
M
nature of photopolymerization has been investigated experimentally via varying the reaction parameters, such as irradiation time, intensity of incident light, monomer concentration and NP
ED
concentration. In most cases, the monomer consumption ([M]/[M0]) basically follows a firstorder dependence of the polymerization rate as shown in Figure 44-a [136]. The polymer yield
PT
increases linearly with the extension of irradiation time, indicating that the rate of
CC E
polymerization was independent of time (Figure 44-b) [284]. The plot of the polymerization rate versus the square root of light intensity is linear, which is in perfect agreement with the kinetic theory shown above (Figure 44-c) [256]. Besides the quality of the NPs, monomer,
A
solvent or co-initiator also may influence the reaction (Figure 44-d). For example, in an early report of Hoffman, the polymerization efficiency of vinyl acetate, methyl acrylate, acrylonitrile, MMA and styrene was assessed using a CdS quantum dot photo-polymerization system. It is found that besides the reduction potential of the monomers, their intrinsic reactivity and polarity played more essential roles in the reaction [282]. The relationship between quantum yield of 58
NPs and dielectric constant of the solvent was also discussed, but such solvent effect may not applicable to other reaction system [282]. Nonetheless, usually the solvent with reduction potential compatible with the energy level of semiconductor NPs, which may work as a good hole scavenger to promote the charge separation, is preferred. Moreover, the choosing of coinitiators needs to be carefully considered. The co-initiators can dramatically enhance the
IP T
efficiency of the polymerization, as they may interact with the surface of the NPs, where photoexcited charge carriers are inherently located [300,329]. The effect of various amine co-
SC R
initiators on the one- and two-photon induced polymerization of MMA using CdS QDs implied that some amines, i.e. TEA, can interact more strongly with trapped carriers than other amines (such as n-butylamine and diethylethlenediamine), although the nature of the interaction is still
U
unknown [329].
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4 Conclusion and Perspective
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In line with the concept of “green” and sustainable chemistry, photocatalytic organic synthesis, e.g. photocatalytic conversion of CO2, aromatic alcohols, fatty alcohols and other compounds
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to fuel, organic molecules and polymers, as one important branch and a frontier in photosynthesis has opened a promising and environmental-friendly venue for the production of
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organics. In this paper, we mainly highlighted the most recent advances about the CO2
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conversion, organic molecule transformation, and photopolymerization initiated by photoactive NPs and. The introduction of photocatalytic CO2 reduction is categorized depending on the catalyst types; and at the end of the introduction we briefly discussed several important
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factors affecting catalysis. The section of photocatalytic organic transform is classified based on the reaction types, and ended with a summary of factors influencing reaction selectivity. In the third part, for the first time, we summarized the application of photoresponsive NPs, which have long been used as photocatalysts in heterogenous catalysis, in the radical polymerization system working as photo-initiators. The semiconductor NPs or nanocomposites are good light59
sensitive materials undergoing photon adsorption followed by the generation of charge carriers, which can induce the formation of initial radicals. The possible reaction mechanism is highly dependent on the type of the NPs, reactants and solvents. Compare with the conventional heterogenous catalysis system, the organic synthesis and photopolymerization initiated by NPs is more complex since monomers, solvents and co-initiators all participate the reaction. In
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addition to the good activity, several outstanding merits of using NP photoinitiator, including,
but not limited to the mild reaction conditions, products with high purity related to the easy
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separation of NPs, and facilely recyclable / reusable NP initiators, make them highly attractive for many applications.
Despite the achievements over the past decades, there are many challenges still existing in
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the domain of photocatalytic organic synthesis. The photoconversion efficiency and selectivity
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for desired products are lower than photocatalytic H 2 evolution or natural photosynthesis. In
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addition to continue developing of highly active catalyst, the exact mechanism needs to be
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systematically investigated, in particular the adsorption and activation of reactants on catalyst
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surface which are playing key role affecting reaction rate and selectivity. Other processing factors determining the reaction selectivity need to be systematically studied. The investigation
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using nanomaterials for photopolymerization is still largely limited in comparison to those in heterogenous catalysis. To further expand the application of photoresponsive nanomaterials in
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radical polymerization, more efforts need to be devoted to the following aspects: Firstly, it is critical to develop more active NP photoinitiators. Regarding the materials, although classic
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semiconductors show good performance in photopolymerizations, there is a large ascendant space to develop high performance photoresponsive nanostructures, such as larger surface area, broader spectral absorption, higher quantum efficiency as well as better charge separation. Nanocomposites containing semiconductors, metal NPs or carbon materials with rationally designed architectures may be promising solutions. Also, it is timesaving but interesting to transfer the recently developed novel nanocatalysts from heterogenous catalysis into the 60
photopolymerization system to assess the reaction efficiency and explore the possible mechanism. Secondly, the radical polymer synthetic system needs to be extended. In addition to increase the study on the controlled / living radical polymerization, NP photoinitiator may also be applied in microemulsion or reverse-micelle polymerization, where more detailed study can be carried out in terms of the effect of surfactant and additives on the photoinitiation. Third,
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co-initiators, such as alkyl alcohol or amines, play an important role to increase the reaction efficiency, as they can react directly with the separated electrons and holes to form active
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species. Therefore, establishment of an optimal partnership between co-initiator and NP photoinitiating dynamics which can favor the reaction synergistically requires numerous attempts. Meanwhile, the influence of solvent on photopolymerization cannot be neglected. The
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last but not least, in the regard of the universality of radical polymerization in practical
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manufacturing, polymerization reactions using a larger variety of monomers are expected. In
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previous study, mainly acrylate chemicals (e.g., MMA) were used as the representative
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monomer only to display the feasibility of the polymerization; however, it is far away from the
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development of the application of the NP photopolymerization system. Therefore, in the future, this may be improved by using nano-photoinitiators to synthesize macromolecules, polymers /
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copolymers with more functionalities and more complex structures.
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Acknowledgements
All authors would like to acknowledge research funding of National Key R&D Program of China (2017YFE0102700) from the Ministry of Science and Technology (MOST). J.Z. is
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grateful for the support from National Natural Science Foundation of China (NSFC, No. 51872128) and Jiangsu University (17JDG008). H.L. thanks the support from NSFC (No. 51732007). H-N.L. thanks the NSFC (No. 21705085) and Jiangsu University (17JDG007). References
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Figure 1. Schematic illustration of the natural photosynthesis (left) and an artificial heterogenous photosynthesis using semiconductor NP as catalyst (right) [13]. Reprinted with
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Figure 2. Proposed mechanism and pathways for the photocatalytic reduction of CO2 with H2O
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vapor to CH3OH and CH4 using TiO2 [49]. Reprinted with permission.
Figure 3. Amount of liquid phase products with time and the schematic consecutive pathway
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for CO2 reduction [59]. Reprinted with permission.
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Figure 4. (a) Transmission electron microscopy (TEM) and high resolution TEM (HR-TEM)
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images of TiO2 nanocubes with single crystalline and exposed {100} and {001} facets. (b) Evolution rate of CH4 and CH3OH using TiO2 nanotubes and other TiO2 catalysts [60]. (c)
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Scanned electron microscopy (SEM) images of the hexahedron prism-anchored octahedronal CeO2 with different prism length. (d) HR-TEM images of the prism. (e) CH4 evolution using
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different CeO2 catalysts. (f) Proposed PET process for photocatalysis using hexahedron prism-
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anchored octahedronal CeO2 [61]. Reprinted with permission.
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Figure 5. (a) Methanol production as a function of reaction time upon different photocatalysts. (b) Plausible mechanism of CO2 reduction on in situ Ru(bpy)3/TiO2 nanocomposite [94].
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Figure 6. Scheme of the two different mechanisms for charge carrier separation: (a)
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conventional heterojunction-type and (b) direct Z-scheme mechanisms [96]. Reprinted with
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Figure 7. (a) Scanned electron microscopy (SEM) image of the SiC@MoS2 nanoflowers. (b) CH4 evolution on the photocatalyst SiC@MoS2 with diferent MoS2 content for 4h reaction. (c) Proposed reaction pathway of photocatalytic CO2 reduction with H2O and SiC@MoS2 [158]. Reprinted with permission
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Figure 8. (a-f) TEM images of the Bi2 WO6-TiO2 binanosheets. (g) Proposed Z-scheme process
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of the photocatalytic CO2 reduction upon the binanosheets [145]. Reprinted with permission.
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Figure 9. Proposed structure modes of CO2 bonding on catalyst [25,191–193]. Reprinted with permission.
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Figure 10. Selective oxidation of p-aminobenzyl alcohol into p-aminobenzaldehyde over
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Au/CeO2 catalyst under green light irradiation [218]. Reprinted with permission.
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Figure 11. HR-TEM images of the Au NPs loaded on the interface of anatase/rutile TiO2. And the proposed process of oxidation of aromatic alcohols on the interface of mixed crystal
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structure [222]. Reprinted with permission
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Figure 12. (a) Reaction scheme of the photoenzymatic hydroxylation of ethyl benzene [223]. (b) Schematic presentation for the synthesis of noble-metal@TiO2-microspheres [225].
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Figure 13. Schematic illustration of the self-assembled heterosupramolecular visible light
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photocatalyst [226]. Reprinted with permission.
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Figure 14. (a) SEM image of the p-BWO nanosheets. (b) Reversible change of p-BWO appearance upon light and O2 presence. (c) Toluene conversion and yields of benzyl alcohol, benzaldehyde and benzoic acid. (d) Conversion rate of toluene oxidation for Bi2WO6, and p-
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BWO at different catalyst loadings [227]. Reprinted with permission.
Figure 15. Aromatic alcohol oxidation over Au-Pd alloy NPs under visible light irradiation
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[229]. Reprinted with permission.
Figure 16. Schematic illustration of the fabrication of CdS(GRM) (M = Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) nanocomposites in which metal ions are introduced to the interfacial layer matrix between GR and semiconductor CdS [233]. Reprinted with permission
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Figure 17. (a) Schematic illustration of the routes to fragment β-O-4 linkage for conversion of lignin. Route A, traditional biorefinery process forming cellulose pulp and technical lignin.
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Route B, lignin-first approach with high-temperature hydrogenolysis upon a conventional thermal catalysis process using supported metal catalyst. Route C, lignin-first approach with
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solar energy-driven photocatalytic process under mild conditions. (b) Scheme of the lightdriven lignin-first valorization of birch woodmeal using CdS QDs as photocatalyst [237]. Reprinted with permission.
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Figure 18. (a, b) TEM images of the WS2/Pd nanohybrids. (c) Photocatalytic Suzuki C-C
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coupling reaction over WS2/Pd and proposed reaction mechanism [238]. Reprinted with
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Figure 19. (Upper) TEM and HR-TEM images of the CdS-MoS2 catalyst and corresponding elemental distribution. (Lower) Schematic illustration of reaction energy profiles via ⋅CH2OH and CH3O⋅ on CdS(100) [239]. Reprinted with permission 95
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Figure 20. (a) SEM images of the nanostructure of Pt@MoO on BVO. (b) EG conversion
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efficiency of the catalysts under visible light irradiation. (c) Proposed mechanism for the
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photocatalytic synthesis of EG over Pt@MoO/ BVO [162]. Reprinted with permission.
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Figure 21. (a, b) TEM and HR-TEM images of the Cu@TiN catalyst. (c) Schematic illustration of the reaction process of the photocatalytic epoxidation of alkenes using Cu NPs as catalyst
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[241]. Reprinted with permission.
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Figure 22. Scheme of reactions of synthesis secondary amines using primary amines as starting materials upon Pt/TiO2 photocatalyst, and a summary of the products (chart) [245]. Reprinted
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Figure 23. Proposed reaction mechanism for the photo-alkylation of aniline over Au–TiO2
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[249]. Reprinted with permission.
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Figure 24. Schematic illustration of the photocatalysis promoted 4-NP reduction into 4-NP
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under visible (a) and UV light (b) irradiation [254].
Figure 25. Photocatalytic formation of imines using TiO2 as catalyst and nitrobenzene as reactant [255]. Reprinted with permission.
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Figure 26. Schematic illustration of quinolines synthesis from nitroaromatic compounds and
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alcohols using arenesulfonic acid functionalized SiO2-TiO2 photocatalyst [262]. Reprinted with
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Figure 27. Ru NP size (left: small and unsupported NPs) and support effect (righ: large and
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Figure 28. Schematic illustration of selective photocatalytic oxidations and reductions of
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mixtures over templated nanocavities [275]. Reprinted with permission.
Figure 29. Amount of products and nitrobenzene selectivity during photocatalytic reaction of aniline
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using Pt/P25 and proposed mechanism of selective oxidation of aniline on Pt/P25 under visible light irradiation [278]. Reprinted with permission.
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Figure 30. Photocatalytic oxidation of benzene and alkylbenzenes by Pt/TiO2 photocatalysts using
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incident light with different wavelengths [279]. Reprinted with permission.
Figure 31. Schematic illustration of photoinduced MMA and BMA polymerization using ZnO (or CdS) [280,281] (a) and Fe2O3 (b) NPs [285], respectively. Reprinted with permission.
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Figure 32. (a) Deconvolution of the DTG curves of PMMA prepared using CdS (left) and AIBN (right). (b) Scheme of the generation of monomer radical using CdS [285]. (c) Two
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possible radical generation mechanisms using TiO 2 [291]. Reprinted with permission.
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Figure 33. (a) Scheme of radical generation and chain growth using dye-sensitized CdS. (b) EPR spectra of different semiconductor system [292]. (c) Transmission electron microscopy (TEM) images of the TiO2 / poly(TBAM-co-EGDMA) core/shell NPs. (d) Bacteria survival (%) after treatment with pristine TiO2, bulk poly(TBAM-co-EGDMA), and TiO2/poly(TBAM-coEGDMA) nanoparticles in the presence and the absence of UV light irradiation. Bacteria
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survival ratio (%) vs. UV-irradiation time on the surface of pristine TiO2 and TiO2/poly(TBAM-
co-EGDMA) nanoparticles against (e) Escherichia coli and (f) Staphylococcus aureus [293].
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Figure 34. (a) Scheme of the photocatalytic polymerization by printing of ethanol amine with TiO2 on a 11-(tricholorosilyl)undercan-1-ol self-assembled monolayer. (b) Atomic force microscope (AFM) image of the patterned PEI brushes. (c) Proposed reaction mechanism of the photocatalytic step-growth of ethanol amine [297]. Reprinted with permission. 103
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Figure 35. Interaction of the photoexcited electron and hole species with different co-initiators
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in a ZnO initiating system [299]. Reprinted with permission.
Figure 36. (a) Illustration of the LA coated Fe3O4 and the amine co-initiator. Proposed mechanism for the photopolymerization of vinyl monomers using LA-Fe3O4 NPs without (b) and with co-initiator (c) [301]. Reprinted with permission.
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Figure 37. Schematic illustration of PEDOT synthesis using CsPbI 3 as the photoinitiator [303].
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Figure 38. (a) Synthesis of ZnO@Ag composite heteronanostructure using P[VBuIm][As]. (b, c) SEM images of ZnO nanoflowers and ZnO@Ag NPs. (d) Conversion ratio of MMA polymerization using ZnO@Ag NPs as photoinitiator. (e) Continuous cycles of polymerization
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using ZnO@Ag NPs [302]. Reprinted with permission.
Figure 39. (a) Photoinduced ATRP using semiconductor NPs as photoinitiators [6]. (b)
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Thickness of the polymer brushes obtained from various monomers after 1 h ATRP reaction. (c) Left figure shows the line traces of the PDMAEMA and PSPMA-PDMAEMA brushes AFM images. The right image is the schematic illustration of the PSPMA=PDEAEMA multilayer
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brushes produced by UV-ATRP [309]. (d) UV-light on-off test during the ATRP of MMA. (e) Gel permeation chromatography (GPC) of the PMMA during chain extension process before (black) and after (gray) sequential monomer addition [310]. Reprinted with permission.
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Figure 40. (a) Scanned electron microscopy (left) and TEM (right) image of NbBA NPs. (b)
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Proposed mechanism of a visible light driven ATRP (b) and RAFT (c) polymerization using
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NbBA as the photoinitiator. (d) Light on-off test of RAFT polymerization of NIPAAm using
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NbBA as photoinitiator. (e) GPC traces of PNIPAAm-b-PDMA (dashed line) and PNIPAAm
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(macro-CTA, solid line) to confirm the living character test of RAFT polymerization using
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NbBA photoinitiator [315,316]. Reprinted with permission.
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Figure 41. Scheme of the UCNP assisted photopolymerization (a) in the presence of UV and
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with permission.
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visible light photoinitiators (b), and the detailed radical formation process (c) [318]. Reprinted
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Figure 42. (a) Structure scheme of the SiO2/RuL and SiO2/Nf/RuL. (b) Dye adsorption ability of SiO2/RuL and SiO2/Nf/RuL. (c) Scheme of the photopolymerization using SiO 2/Nf/RuL as
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photoinitiator [319]. Reprinted with permission.
Figure 43. Schematic mechanism of the photopolymerization under UV (a) and visible light (b)
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using light responsive Au NP-BST [326,327]. TEM images show the Au NP-BST before and after light irradiation without monomers [326]. (c) Scheme of the plasmon initiated
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polymerization and the scanned electron microscopy (SEM) images of Au NPs before and after
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irradiation, showing the formation of polymer [328]. Reprinted with permission.
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Figure 44. (a) Monomer consumption (ln([M]/[M0])) vs. reaction time [136]. (b) polymer yield
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as a function of irradiation time [284]. (c) Polymerization rate vs. Square root of light intensity
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[256]. (d) Co-initiator effect on the polymerization [329]. Reprinted with permission.
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S0926-3373(19)30195-X https://doi.org/10.1016/j.apcatb.2019.02.071 APCATB 17506
To appear in:
Applied Catalysis B: Environmental
Received date: Revised date: Accepted date:
15 October 2018 31 January 2019 25 February 2019
Please cite this article as: Liu M, Peng T, Li H, Zhao L, Sang Y, Feng Q, Xu L, Jiang Y, Liu H, Zhang J, Photoresponsive Nanostructure Assisted Green Synthesis of Organics and Polymers, Applied Catalysis B: Environmental (2019), https://doi.org/10.1016/j.apcatb.2019.02.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoresponsive Nanostructure Assisted Green Synthesis of Organics and Polymers Maosong Liu,a Tingyu Peng,a Henan Li,a Long Zhao,a Yuanhua Sang,b Qunwei Feng,c Li Xu,d
a
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Yinhua Jiang,a Hong Liub,e* and Jianming Zhanga*
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013,
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China.
State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China.
c
Jiaxing Rejdue Environmental Technology Co. Ltd, Jiaxing, 314006, China.
Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu
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d
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b
Institute for Advanced Interdisciplinary Research (IAIR), University of Jinan, Jinan 250022,
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e
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University, Zhenjiang, 212013, China.
China.
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Graphical abstract
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Corresponding author email: [email protected], [email protected]
Highlights
Review of recent advances in heterogenous photocatalytic organic synthesis. 1
Photoresponsive nanostructure assisted conversion of CO2 to organics.
Heterogenous photocatalytic organic conversion.
Photoresponsive nanostructure initiated radical polymerization.
Nanoparticle initiators for photosynthesis.
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Abstract
Solar light is believed to be the most sustainable and clean energy source. In line with the
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concept of green chemistry, the use of solar energy has been a formidable impetus toward the
development of novel photo-based synthetic technologies to drive various chemical reactions. Photosynthesis is a promising route to achieve a wide array of chemical transformations with
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distinctive energy and environmental merits which are usually inaccessible with conventional
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thermal processes. Photoinduced organic synthesis, as one important branch of photosynthesis,
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has attracted increasingly attentions to meet the growing-up environmental and energy concerns.
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Highly photoresponsive nanoparticles (NPs), such as semiconductors, can generate photoexcited charge carriers, i.e. electron-hole pairs, upon photon absorption, which can favor
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a vast number of reactions, demonstrating unique advantages including the easily recycling and
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reuse, the use of mild reaction conditions and the generation of high-purity products devoid of contaminants. This review article highlights recent representative advances in heterogenous
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photocatalytic organic synthesis, mainly including CO2 reduction, organic transform and free radical polymerization; in particular, for the first time, the achievements on photoresponsive NP initiated free radical polymerizations are summarized here. We conclude this review by
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proposing several interesting research directions and future challenges with the hope that it can serve as a good reference for researchers in nanomaterials and catalysis. Keywords: Heterogenous photocatalysis, CO2 reduction, Photo organic synthesis, Photo polymerization, Nanoparticles 1 Introduction 2
Today, synthetic organic chemistry constitutes a crucial part of human endeavors in chemical science, which offers us materials in huge diversity, such as fuels, biomolecules and plastics. Typically, to obtain desired chemicals with high production yield or drive synthesis at high reaction rate, harsh reaction / operation conditions, e.g. high temperature and / or pressure, are usually necessary in many synthetic processes, which usually are driven by fossil energy. The
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over-consumption of the fossil fuels has caused excessive emission of carbon dioxide (CO 2) and other toxic byproducts, which produce serious pollutants to the environment. The
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increasing environmental and sustainable-development requirements have thus impelled chemists to explore advanced “green” and clean routes for organic synthesis [1–4]. The ecosystem on the Earth is initiated and supported by the super power source of the sun,
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almost without exception. Upon the acceptance of the solar energy by plants and other light-
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harvesting organisms, chemical transitions are initiated to form new chemicals to make
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creatures vital and diverse. Such a “green” and clean process, known as the natural
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photosynthesis, utilizes sunlight and carbon dioxide (CO2)/water as a sustainable energy source
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and cheap raw materials, respectively (Figure 1 left). Inspired by the natural photosynthesis of plants, artificial photochemistry technology has been intensely investigated for chemical
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synthesis [5–10]. In principal, a photoreaction is triggered by a photon-electron-transfer (PET) process, where upon photon energy absorption electrons at molecule orbitals enter the excited
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states and become highly active to promote chemical transformations. Compared with its thermal counterpart, photosynthesis demonstrates a distinctive advantage to meet the energetic
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requirements in conducting reactions. Using incident light as energy source, a large variety of chemical reactions can be initiated to selectively catalyze chemical transformations which would be difficult to be activated thermally [5–10]. Solar energy is without contest the greenest and most sustainable energy source currently accessible. Therefore, photochemistry based on solar light is a sustainable, eco-friendly chemical process which has attracted significant attention. Using solar energy to directly convert CO2 and water into carbon-containing fuels or 3
chemicals is a major topic and frontier in synthetic chemistry. Based on the recognition of the photo-responsive molecules, such as organic dyes or transition metal chromophores, photoinduced organic synthesis from small molecule to polymer scale has been established and rapidly developed for more than a century [5–10]. Nonetheless, many drawbacks still exist in
molecules are toxic and may remain in products as impurities [11,12].
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the photo-responsive molecule-initiated synthesis; for example, many of the photo-responsive
Initiated by the seminal work of Fujishima on 1979 [14], the research of heterogenous
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photocatalysis has led to fruitful achievements, and is still in a rapid growth stage. Numerous
types of photoresponsive nanoparticles (NPs) have been created and intensely studied [13–21]. As illustrated in Figure 1 (right), in typical heterogenous photocatalysis, the absorption of
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photons excites electrons from valence band (VB) to conduction band (CB) of the
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semiconductor NP (e.g. TiO2), leading to the generation of electron-hole pairs (e-/h+) or charge
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carriers. The so-generated electron-hole pairs immigrate to the surface of the NP to interact
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with electron acceptors and donors adsorbed on NP surface via the PET process, forming
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extremely active reducing and oxidizing species respectively. The synthesis, therefore, can be proceeded through either oxidative or reductive pathways, or through the combination of both
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[13,19]. From view point of green chemistry, semiconductor NPs show desirable properties as photoreaction initiators [13,16,18], where the structures and components can be intendedly
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designed and tuned, using a plethora of known methods, to obtain better charge separation and higher quantum efficiency. More interestingly, compared with the homogeneous photoreactions
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using small molecules (e.g., dyes), NPs are more chemically stable, environmentally benign as well as less toxic. In addition, as the NPs are insoluble in reaction solution, they can be easily recycled / reused in the successive reactions without remaining in the products, thus leading to a generation of clean products in “one-pot” reactions [13–21]. The pioneer work by Inoue [14] using semiconductor as photocatalyst has opened a revolutionary venue to realize the conversion of CO2 into simple carbon fuels or organics, e.g. 4
HCOOH, CH3OH, HCHO and CH4, upon solar light as the sustainable energy source, revealing a promising route to reduce greenhouse gas and simultaneously produce an array of useful organics in a green fashion and leading to a vast number of publications. Significant achievements have been obtained in the evolution of reaction efficiency as well as the understanding of the reaction mechanism. Some excellent reviews have summarized the-state-of-the-art of heterogenous
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photocatalytic CO2 reduction applications [22–28]. For instance, Gong et al. [25] reviewed the main strategies to improve the surface interaction (adsorption/activation) of CO2 molecules on
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semiconductors and the factors dominating the reaction pathways. Jaroniec et al. [26] summarized
various 3D hierarchical photocatalysts including nano / microspheres, hetero-structured nanocomposites, yolk/shell structures and hollow structures etc., bearing the advantages of large
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surface areas, high CO2 adsorption capabilities and high mass transfer rates for catalysis. Zhang et
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al. [29,30] reviewed the CO2 reduction using perovskite oxide nanomaterials, and pointed that
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perovskite oxides exhibit wider absorption, longer charge carrier lifetime, and favorable band-
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alignment for CO2 reduction, which is supposed to be more active than conventional semiconductors.
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Yu et al. [31] have reviewed the application of graphene (GR) materials in photocatalytic CO2 reduction, showing that due to the unique properties including excellent conductivity, good optical
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transmittance, large specific surface area, and superior chemical stability, GR-based catalysts demonstrate good activity. The daily-updated reports enrich the fundamental understanding as well
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as the development of highly efficient catalysts. Although abundant scientific results in this domain, currently heterogenous photocatalytic
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CO2 reduction is mainly applied to produce small hydrocarbons. To produce more complex or value-added organics, heterogenous photocatalytic organic conversion has been developed. Upon the presence of photo-generated active radicals, redox reactions can be conducted to small organic molecules, such as alcohols or alkenes, to produce corresponding derivates or new chemicals under a mild reaction condition. Some reviews on photocatalytic organic conversion have been published [23,24,40,41,32–39], of which several are more specific on certain 5
reactions. For example, in a review by Yao et al. [37], photocatalytic oxidation reactions, especially, alcohols oxidation, aromatic hydroxylation and alkene oxidation are summarized. Xu and Luque et al. [38,42] reviewed the advances in the selective heterogenous photocatalytic valorisation of lignin-based compounds. Li et al. [24] surveyed the work on oxidation of amines, carbon-carbon bond formation and carbon-hetero bond formation. Bahnemann and co-workers
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[23] reviewed the reaction selectivity in photocatalytic organic synthesis, and pointed that the screening of photocatalysts for specific reactions may reach better selectivity. Nonetheless, the
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fundamental understanding on the exact reaction mechanism is still lacking.
Polymers or macromolecules can also be obtained via a heterogenous photocatalytic process. Very recently,
heterogenous photocatalytic polymerization technique has
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demonstrated an evolution to its traditional thermal counterpart to meet the growing demand
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for easy curing, highly functionalized and well-defined polymer structures as building blocks
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in nanotechnology, biological and 3D fabrication applications [43,44]. A conventional free
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radical polymerization process usually works at high temperature, where initial radicals are
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generated by thermal decomposition of molecule initiators (e.g. organic/inorganic peroxides or diazo compounds) or upon reacting redox-active compounds [45–48]. Despite their remarkable
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ease of use, their low cost and wide availability, these initiators also show several drawbacks due to their residual presence in the polymer, their lack of sustainability, their odor and potential
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toxicity. Heterogenous semiconductor NPs show desirable properties as photoinitiators [13,16,18], since the photogenerated charges can result in the formation of highly active radicals
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to initiate the reaction. However, compared with the intense focus of CO2 reduction and organic conversion, the study on heterogenous photocatalytic polymerization receives less attention, and the review with a systematic summary of the advances in this research domain is missing. As a complementary to previous excellent reviews, here we have reviewed some up-todate reports with high novelties from the most prestigious journals. We mainly introduce the application of NP catalyzed photoradical organic synthesis. We start by highlighting the recent 6
advances of semiconductor NPs for photocatalytic CO2 reduction and move to organic conversion, including alcohols oxidation, aromatic hydroxylation and so on. To differentiate from other review articles as mentioned above, we mainly introduce some very recent state-ofthe-art achievements and give a summary of the key parameters affecting the efficiency and/or selectivity of reactions, which are highly diverse yet have been strongly concerned in this field.
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Then we move to the section of photoinduced free radical polymerization with more detailed
information in a relatively longer length, which is systematically summarized for the firstly
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time; in particular, latest achievements on the use of inorganic NPs for controlled or living radical photopolymerizations are introduced. The fundamental mechanisms underlying the
reactions are also briefly addressed. This review concludes with challenges and proposals on
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how to broaden the utilization of photoresponsive NP in organic transforms, in particular for
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polymerization, and how to gain more fundamental understanding on the mechanism of
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photosynthesis. We expect that this review could be a valuable reference for the readers who
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are interested in photocatalytic synthesis for environmental and clean energy generation.
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2. Photoresponsive NP Catalyzed Organic Synthesis 2.1 Photocatalytic Conversion of CO2 to Organics
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As aforementioned, reports about heterogenous photocatalytic CO2 reduction have been
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emerging numerously. Nowadays, to obtain high performance, most of the photocatalysts have been evolved into hetero or hybrid nanostructures, which are mainly composed of several different components with multi-functions or rationally designed architectures. Simple and
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classic semiconductors, such as TiO2 or CeO2 NPs, are seldom used alone for efficient photocatalysis due to their intrinsic drawbacks, e.g. low absorption and quantum yield; however, they are still active for mechanism study and employed as good platforms to fabricate more complex structures. In this section, the content is sequenced on the basis of catalyst type. We start from the most classic TiO2 by reviewing several latest innovations of simple 7
semiconductor system; afterward we introduce some recent advances of modified semiconductors and nanohybrids. At the end, the parameters affecting reaction efficiency are discussed briefly. 2.1.1 Simple semiconductor catalysts (TiO2, CeO2 and so on) TiO2 NPs, as the most intensively investigated photocatalyst, have been widely applied to
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catalyze the reduction of CO2 into fuels and hydrocarbons because of their excellent UV light
absorption, chemical stability, low cost and easy fabrication etc. [49]. Through precisely
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programming the microstructures of TiO2 NPs, such as porosity, oxygen vacancy or crystallite
structure, an idea platform can be created to investigate the structure–activity relationship to gain more fundamental understanding on catalysis. Oxygen vacancies (OVs) have been
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acknowledged to play a vital role in many photocatalytic reactions [50–52]. The OVs at surface
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or bulk show distinct functions for catalysis [50–52]. To better understand the effect of OV on
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catalysis, Yang et al. [53] prepared three different TiO2 NPs with surface OVs (SOVs) and/or
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bulk single-electron-trapped OVs (SETOVs) via simple approaches of dehydration or reduction
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of different Ti precursors. Photocatalytic CO2 reduction test under Hg lamp irradiation showed that both the bulk SETOVs and SOVs contribute to light absorption enhancement. The SOVs
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facilitate charge-carrier separation, while the bulk SETOVs work more as the recombination center. For the case of the co-existence of the surface and bulk OVs, the catalytic efficiency to
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convert CO2 to CH4 can be improved by increasing the ratio of surface OVs to bulk SETOVs, indicating a synergistic effect for catalysis. The presence of OV-related defects could not only
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act as the activation sites for catalysis [53–55], but also can enhance the light absorption of TiO2 to the visible range [56]. Using KIT-6 SiO2 as template, Russo et al. [57] synthesized mesoporous TiO2 NPs with high surface area (190 m2/g vs. 53 m2/g of P25). CO2 reduction tests showed that the catalysis upon this mesoporous TiO2 led to more hydrocarbons, better reaction kinetics and higher stability, due to its superior characteristics than those of the conventional TiO 2 NPs. The larger 8
surface area of mesoporous TiO2 is highly beneficial to lower adsorbing energy barrier of reactants (CO2 and H2O) to TiO2. A recent study using mesoporous TiO2 NP for CO2 reduction [49] proposed that that upon photon absorption, photo-excited electrons reduce the adsorbed CO2 to an anion radical CO2•−, while H2O oxidation is preceded through the interaction with holes, leading to the formation of H+ and OH• radicals, as demonstrated in Figure 2. H+ may
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accept electrons to form H•, which then reacts with CO2•− to form CO, CH•, CH2• and CH3•.
Subsequently, these alkyl radicals interact with OH• to form CH3OH as a product with a
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combination of CH4 and O2 [24]. The reaction proceeds mainly by the competitive adsorption
of CO2 and H2O on the surface of TiO2 NPs. In addition, UV light intensity, H2O/CO2 ratio, and structure of NPs all exert important impact on the performance of the catalyst and product
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yield. The saturation of adsorption center may induce an undesired reverse reaction, where the
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produced methanol reacts with oxygen (O2) to generate CO2 [49]. Kang and co-workers [58]
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also synthesized ordered mesoporous metal oxides (TiO2 and SnO2) with high surface areas
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(~100 m2/g) for CO2 photoreduction. Owing to their high surface area, the mesoporous metal
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oxides demonstrate 9 to 10 times higher yields of CH4 and 2 to 3 times higher yields of CO than P25.
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Rossetti et al. [59] studied the mechanism of photoreduction of CO2 at high pressure in
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liquid phase using P25 as photocatalyst in the presence of Na2SO3 as hole scavenger working at different pH. Using a special reactor which allows improving CO2 solubility in water at high pressure, products of liquid phase (formic acid, formaldehyde) and gas phase (CH 4) can be
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successfully produced. Reaction time investigation showed that at the beginning, liquid product of formaldehyde was the main product, followed by the formation of formic acid. Interestingly, as the extension of reaction time, liquid phase products reduce gradually (Figure 3, upper), while gases, mainly composed of H2 and CO, start forming and become the major product after the consumption of the Na2SO3, which can be ascribed to the photodegradation of the pre9
formed organic products (Figure 3, lower). At this stage, the organic species formed at the beginning, e.g. formaldehyde, may act as alternative hole scavengers. By optimizing reaction condition, such as temperature, pH and reaction time, the productivity of formaldehyde and formic acid can reach as high as 16.537 mol h−1 kgcat−1 and 2.95437 mol h−1 kgcat−1, respectively. Recent study shows that anatase TiO2 nanocubes with single crystalline and exposed {100}
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and {001} facets, synthesized by hydrothermal and calcination approach, display significantly
high photoactivity in CO2 reduction to CH4 and CH3OH, because of the synergetic effects of
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highly ordered crystallization, co-exposal of {100} and {001} and more negative CB position
facets (Figure 4 a and b) [60]. CeO2 NPs have also been broadly used as photocatalyst in CO2 reduction. A novel CeO2 homojunction nanomaterial with unique crystallite structures has been
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fabricated via a crystallographic-oriented growth method by Zhou and co-workers [61], as
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shown in Figure 4 c-f, where hexahedron prism with exposed {100} facet was anchored on
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octahedronal with exposed surface of {100} facet. The inert CeO2 octahedron can be activated
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by prism arms on the surfaces, leading to an increase of its photocatalytic activity for the
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reduction of CO2 into CH4. Three possible factors were proposed to interpret the significantly enhanced performance: first, the prism arms act as fast channels for the transportation of the
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photogenerated electrons of the homojunction; second, the band alignment of the two exposed facet of {100} and {111} favors the charge separation to the surface of the catalyst; third, high
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charge carrier mobility due to the different effective mass of electrons of {100} and {111} facets greatly facilitates the charge separation.
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2.1.2 Modified semiconductors In addition to simple metal oxide NPs, compound semiconductors, such as g-C3N4, ZnS, CdS and CdSe have also been used for photocatalytic CO2 reduction [23,24,32–41,62–69,70–75]; however, the photoactivity is restricted due to their deficient inherences, such as bandgap limit or high charge recombination rate. Therefore, a great number of efforts have been conducted to
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increase the absorption and enhance the quantum efficiency of conventional semiconductors, including doping, dye-sensitizing and forming heterostructures or cocatalysts. 1) Doped semiconductors The introduction of dopant, either metals or non-metal elements into semiconductor lattice can effectively suppress the recombination of photogenerated charge carriers, rapidly transfer
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electrons/holes to the adsorbed reactants, and correct the band gap structure, even by the
concomitant red shift of the intrinsic absorption edge. For instance, doping TiO2 with N, S and
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Ni2+ [76–78] can narrow the band gap and thus increase the visible light absorption. Yu et al. [79] synthesized S-doped g-C3N4 with wider absorption range up to 475 nm. Photocatalytic
CO2 reduction test showed that the CH3OH production yield using S-doped g-C3N4 is much
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bigger than using undoped g-C3N4 (1.12 vs. 0.81 μmol g−1). Recently Yu also reported O-doped
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g-C3N4 nanotubes as photocatalyst for CO2 reduction with improved performance [80]. Besides
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the modification to band gap and charge separation efficiency, the O-doping to g-C3N4 enhances
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the affinity and uptake capacity to CO2. TiO2 NPs doped with Indium (In) exhibits increased
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surface area and enlarged band gap [81]. The photocatalytic efficiency of In-doped TiO2 NPs was greatly improved for CO2 reduction. CO was the main product over pure TiO2, whereas
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CH4 yield was considerably increased using In-doped TiO2. This enhancement is mainly attributed to the largely increased interfacial transfer efficiency of photogenerated charges,
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leading to effective charge separation and suppressed recombination of photogenerated charge carriers, owing to the structural change induced by the insert of In into TiO 2 lattice. Ola and
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Maroto-Valer [82] reported that the Cu doped TiO2 show visible light absorption with enhancing Cu loading concentration in comparison with pure TiO2. This change is ascribed to the interaction of Cu+ ions with TiO2 crystal lattice via the replacing Ti4+ with Cu+; moreover, these Cu+ species can act as electron traps to suppress the charge recombination and favor the multielectron reactions, thus largely improve the CO2 reduction efficiency. Other transition
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metal elements (such as Fe, Co, Pt, Mo and Ni etc.) doped TiO2 are also found to be effective photocatalysts for reducing CO2 to CH4, and other hydrocarbons [81–92]. 2) Sensitized semiconductor The activity of semiconductor NPs can also be enhanced by sensitizing their surface with photoresponsive molecules, e.g. organic or metal complex dyes, to surface of the
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semiconductors. In this system, the photogenerated electrons can transfer between sensitizer
and CB of semiconductor, and the transfer pathway is determined by the relative positions of
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the highest occupied molecular orbital and lowest unoccupied molecular orbital
(HOMO/LUMO) levels of dyes and CB of semiconductors. In most cases, the dye molecule acts as electron donor and semiconductor as electron acceptor during photocatalysis [93]. The
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semiconductor with only UV light response, such as TiO2 or ZnO, can be easily modified into
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a visible-light responsive one using suitable sensitizers, Metal complex molecules are more
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stable than organic dyes, and they are thus widely used as sensitizers in photocatalysis. As
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presented in Figure 5, Pawan et al. [94] reported a Ru(bpy)3Cl2 sensitized TiO2 nanohybrid
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(Ru(bpy)3/TiO2) as highly selective catalyst for photocatalytic reduction of CO2 under visible light. In this system, Ru(bpy)32+ component absorbs visible light and transfer electrons to the
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CB of TiO2 for reaction. It is found that the visible light-driven methanol production yield upon Ru(bpy)3/TiO2 hybrid catalyst was much higher than that using pure TiO2 catalyst and
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Ru(bpy)3Cl2 alone. P25 has also been sensitized using Cu complex of Cu(II) tetra(4carboxylphenyl)porphyrin (CuTCPP) using a hydrothermal approach [95]. CH4 production in
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potocatalytic CO2 reduction using CuTCPP/P25m (0.5 wt % of CuTCPP) is approximately 46 times higher than that using P25 alone, which is mainly attributed to the Cu sensitizer, which enhances the absorption of visible light and the rate of charge carrier separation. In another report [33], a comparison study using different sensitizers of Cu(II) phthalocyanines (CuPc) and Cu(II) porphyrins showed that the Cu(II) phthalocyanine(CuPc) (Cu(II) co-ordinated in the middle of the macrocycles) is highly beneficial for the photoactivity, probably due to its 12
favorable reduction potential, indicating the important role of sensitizer structure on catalytic activity. Currently many attentions have been focused on phthalocyanines containing center metals including Zn(II), Cu(II), Co(II), Ni(II) as sensitizers of semiconductors [96,97]. 3) Heterostructures or hybrids Without doubt, today almost entirely of efforts in this research domain are concentrated to
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rationally design semiconductor heterostructures or nanohybrids to improve the reaction
efficiency. The CO2-reduction heterostructure catalysts can be generally divided into three
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categories: noble metal-semiconductor hybrid catalysts, noble metal-free hybrid catalysts, and
multi-semiconductor hybrid catalysts, among which most of the attention are focused on the noble metal-semiconductor heterostructures, leading to the huge increase of publications daily.
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As there have been some excellent reviews about this topic, here we mainly summarize some
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a) Noble metal-based hybrid catalyst
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very recent and most representative work.
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Noble metal NPs, such as Pt [98–104], Pd [105,106], Ag [107–110], Ru [111,112], Au [113]
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and alloy (or bimetallic) NPs [114–119] have been broadly applied to conjugated with semiconductors to form heterostructures. The synthesis to integrate metal NPs with
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semiconductor can be generally divided into two methods, which are the in-situ growth of metal along with semiconductor, and the coupling of as-synthesized components. Composition,
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morphology, structure, crystal facets, size, and alloy phase of the noble metal NPs demonstrate profound effect on the activity and selectivity of photocatalytic CO 2 reduction.
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Among all noble metals, Pt possesses the unique feature of the highest work function of
5.56 eV, suggesting a very low Fermi level and strong electron-accepting ability. Therefore, Pt nanostructures have been largely applied to decorate semiconductor surface to promote separation of photo charge carriers, thus enhancing catalytic efficiency [28]. Pt NP decorated TiO2 (Pt/TiO2) is the most intensively investigated system for photocatalytic CO2 reduction. For example, very recently, Tesbihi and Fresno [102] reported a mesoporous SiO2 supported 13
Pt/TiO2 catalyst, which was synthesized by classic deposition-precipitation method. CO is the main product with only tracible CH4 and CH3OH using TiO2 alone as the photocatalyst, while using Pt/TiO2, CH4 becomes the major product with a selectivity of ~100% with optimum Pt loading. In-situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) characterization indicates that the Pt NPs on TiO2 can strongly chemisorb the CO intermediate,
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thus preventing CO to be the main product and further promoting the total reduction of CO 2 to CH4. Zhang et al. [103] loaded Pt and Cu2O on TiO2 surface for CO2 reduction. In the hybrids,
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Pt usually tends to promote the production of CH4 with undesired H2. On the other hand, Cu2O is an effective material to suppress the production of H2; however, it shows lower CH4
selectivity than that of Pt. Thus, the combination of both with an appropriate ratio can largely
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improve CH4 production. It is found that Pt shows dual functions to accelerate separation and
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transport of photogenerated electrons, and increase the electrons density of Cu2O NPs, which
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is beneficial for selective CH4 formation. In another report of Pt/In2O3 system, researchers [104]
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found that due to electronic and steric effect, Pt NPs can modulate the surface feature of In2O3,
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reducing the formation of bicarbonates (HCO3−), bidentated carbonate (b-CO32−) and monodentated carbonate (m-CO32−) species toward the production of HCOOH, CH2O and
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CH3OH. More importantly, Pt play a key role in the selective formation of CH 4, as it weakens H-Pt bond and facilitate the bonding of C in CO2 with protons to form CH4, which
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simultaneously restrains CO production. As the most studied catalyst, Pt has been widely employed to couple with various semiconductors, such as g-C3N4, CeO2, Zn2GeO4, LaPO4
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[61,99,120,121], and composites of In2O3/g-C3N4, NaNbO3/g-C3N4, HNb3O8/SiO2, CdSe/TiO2, CdS/SiO2 etc [74,100,101,122,123], to improve reaction activity and selectivity. Pd has also been decorated to semiconductors to improve reaction efficiency of photocatalytic CO2 reduction. Through a density functional theory (DFT) study, Liu et al. [124] had proposed that Pd/In2O3 is a promising catalyst with high activity for CO2 reduction to methanol, because Pd is expected to improve the catalysis by offering metal sites for H2 14
dissociation-adsorption and interfacial sites for CO2 adsorption-hydrogenation, though there is still challenge experimentally. Very recently, they have synthesized the Pd/In2O3 hybrid via a thermal decomposition method, which successfully prevented the formation of PdIn alloy at composite interface [125]. Highly-dispersed small Pd-NPs are exposing predominately the {111} facets on In2O3 surface. The Pd NPs demonstrate better ability to adsorb–dissociate
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hydrogen, thereby providing active H for the hydrogenation and accelerating the generation of oxygen vacancies on semiconductor surface. The Pd-In2O3 interfacial sites are also vital to
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promote CO2 adsorption and hydrogenation. As a result, a superior performance of the Pd/In2O3
catalyst for CO2 reduction to methanol is reached with a CO2 conversion >20% and methanol selectivity >70%. Bai et al. [105] investigated the relationship between Pd NP facet and
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catalytic selectivity, by loading single-crystal Pd nanoclusters (NCs) or Pd nanotubes (NTs)
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enclosed by {100} and {111} facets on g-C3N4 nanosheets. The Pd/g-C3N4 samples all showed
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enhanced charge separation efficiency; however, they found that the Pd NT/g-C3N4 showed a
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much higher selectivity (80%) than that of Pd NC/g-C3N4 (20%) in CO2 reduction to CO,
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C2H5OH, and CH4. This facet selectivity relationship is attributed to the higher adsorption energy (EA = 0.23 eV) and lower activation energy barrier (EB = 3.98 eV) on Pd {100} than
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those on Pd {111}, respectively.
Au, Ag and Cu NPs are well-known materials as cocatalyst for visible light driven
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photocatalytic water splitting, due to their unique optical property of surface plasmon resonance (SPR). These plasmonic nanohybrids have also been used for CO2 reduction. Au NP is deemed
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to be the most famous SPR material with strong and tunable visible to near infrared absorption. Various Au/TiO2 photocatalysts are used for CO2 reduction under visible light irradiation [126,127]. For example, mesoporous Au@TiO2 yolk−shell hollow spheres with highly crystalline and thin TiO2 has been reported by Zou’s group [127]. The SPR-induced local electric field at Au/TiO2 not only enhances the separation of photo charge carriers in TiO2 shells to improve the CO2 reduction rate but also favors the formation of high-grade carbon species 15
(C2H6), which is rarely reported in CO2 reduction systems. They also reported the Ag/TiO2 catalyst for efficient CO2 reduction owing to the SPR and electron sinker effect of Ag component, which is closely related with the amount and size of loaded Ag. In this system, diverse hydrocarbons were produced, mainly including CH4 and methanol [127]. Cu NPs have also been hybridized on TiO2 surface (Cu/TiO2) to increase the production yield of CH3OH, as
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they exhibit a dual role as electron sink to improve the photo-charge separation and light
harvesters to enhance the visible light absorption due to their localized SPR [128,129]. Besides
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the SPR property, these noble metals also show catalytically active sites in reactions. Small Ag
NPs have been loaded on ZnGa2O4, ZnTa2O6, SrNb2O6 and Sr2Nb2O7 for photoreduction of CO2 [130–132]. Tanaka et al. [131] proposed the reaction mechanism on Ag/SrNb2O6 nanorod,
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where the reduction of CO2 led to the formation of CO mainly on the top of the Ag/SrNb2O6
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nanorods, while O2 was generated mainly on the sides of the nanorods. The separation of the
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reduction and oxidation sites greatly suppressed the recombination of the photogenerated
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charge carriers; Ag-loaded SrNb2O6 nanorods thus exhibit good activity and selectivity. In
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another report, Vajda el al. [133] found small Cu clusters on Fe2O3 surface favor the activation of hydrogen, which can spill over to the neighboring Fe sites, and thus enhance the conversion
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CO2. Other Ag based compound catalyst, such as AgBr NPs have also been used for efficient CO2 reduction [134].
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Recently, Ru has attracted increasing attentions as a promising replacement to expensive
noble metals (i.e. Pt), as they show good activity and stability if rationally designed and more
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importantly price advantage compared with Pt [112,135–137]. For example, Zhang et al. [135] studied the effect of different oxide support (Al2O3, CeO2, MnO2 and ZnO) of Ru based catalysts on CO2 reduction. For the pristine oxides alone, CO was the major product due to a lack of H2 dissociation sites on these oxides. The addition of Ru created H2 dissociation sites and led to a significant increase of CO2 conversion and CH4 selectivity. The ZnO support resulted in the formation of lowest Ru-CO coverage with a weak CO adsorption and the reverse 16
water-gas shift was the dominant reaction. CeO2 showed a moderate Ru-CO coverage in between Al2O3 and ZnO, which favors the creation of H2 dissociating sites and remains the high Ru-CO adsorption strength. Zeng et al. [136] reported a mesoporous silica nanowire encapsulated Ru NP catalyst (Ru@mSiO2) for CO2 reduction. CO was the main product for 13 nm Ru NPs within mesoporous silica nanowires, while CH4 was the major one for bigger Ru
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NPs (5-20 nm). This difference is mainly ascribed to the different reaction intermediates formed on Ru@mSiO2 catalyst surface: CO-Run+ on 1–3 nm and formate species on 5–20 nm.
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Bimetallic NPs, including alloys, core-shell and intermetallic structures, usually contains at least one noble metal, which have demonstrated the so-called synergistic effect on promoting the photocatalytic CO2 reduction. In most cases, these bimetallic NPs are distributed on
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semiconductor surface as cocatalysts for photoreactions. For example, Song et al. [138]
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reported a SiO2 supported Pd-Cu alloy NP catalysts for selective CO2 hydrogenation to
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methanol. In the alloy structure, the component ratio of Pd could be varied in the range of 25–
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34 mol %. It was found that the alloy with 25 mol % Pd (Pd(0.25)–Cu/SiO2) demonstrated the
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highest activity with a methanol formation rate twice higher than the simple sum of monometallic Cu and Pd catalysts, indicating a synergistic effect for catalysis. It is proposed
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that the alloys could provide a source of chemisorbed hydrogen atoms, which favors the formation of CH3OH. Zhao and co-workers [116] reported a three-dimensionally ordered
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microporous (3DOM) TiO2 supported Au-Pd core-shell-like NPs. Since bimetallic AuPd nanoparticles with the relatively low Fermi level have good capacity of trapping electron, they
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can efficiently promote the separation of photogenerated electron-hole pairs in TiO2. The AuPd/3DOM-TiO2 catalysts demonstrate excellent activity for photocatalytic CO2 reduction. In particular, Au3Pd1/3DOM-TiO2 catalyst exhibits the highest activity and selectivity for CO 2 reduction to CH4 with a formation rate of 18.5 μmol g -1 h-1 and selectivity around 93.9 %. In photocatalysis, upon light absorption, the excited electrons in CB of TiO2 are transferred to Au NPs. And then the electrons further transfer from Au to Pd, which further decreases the 17
recombination rates of electron-hole pairs. Electrons assemble on metal NPs react with the adsorbed CO2 to form CH4, while holes on the VB of TiO2 oxidize H2O. Therefore, Au-Pd coreshell NPs not only offers the reduction sites but also serves as the electron sinks, leading to an effective charge separation and photocatalytic activity enhancement. AuFe, AuAg and Pd-In based catalysts have also been reported and show large activity enhancement for CO2 reduction
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into hydrocarbon products [118,119,139]. b) Noble metal free catalyst
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Compared with the noble metal-based catalyst, non-noble metal one exhibits superior cost
advantage, implying higher feasibility for mass-production or commercialization. Ni is a famous material as non-noble metal cocatalyst for various catalytic reactions, including CO2
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reduction [140–142]. For instance, Dai et al. [143] reported a Y2O3 supported Ni NP catalyst
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as photocatalyst for CO2 reduction. By changing different synthetic methods, the authors found
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the approach using YO(NO3) acting as a unique matrix to afford anchoring sites to interact with
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Ni2+ ions, can result in a moderate interaction between Ni metal and Y2O3 support, thus inducing
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excellent catalytic activity and stability towards CO poisoning in photocatalytic CO2 reduction. Garcia and Albero [130] synthesized GR supported NiO/Ni NPs (NiO/Ni-GR) as efficient
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photocatalyst for gas phase CO2 reduction with H2. The hybrid with an optimal Ni loading of 23 wt % showed the highest activity with the maximum specific CH4 formation rate (642 μmol
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gNi−1 h−1 at 200 °C). Under the same conditions, Ni NPs supported on SiO 2-Al2O3 or NiO NPs alone only showed much lower CH4 production rates than NiO/Ni-GR. In another report, the
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presence of Ni@NiO core@shell co-catalyst increased the photoactivity of N doped InTaO4 (N-InTaO4) for CO2 reduction to CH3OH under visible light irradiation [30]. Zou’s group [144] reported a KOH-modified Ni/LaTiO2N photocatalyst for visible light CO2 reduction, showing high performance in CO2 reduction with CH4 generation rate of 9.69 μmol g−1, about 5 times higher than that upon LaTiO2N. The significant activity enhancement is mainly attributed to a formation of Schottky barrier at Ni/LaTiO2N interface boosting the separation of electron-hole 18
pairs. The OH- of KOH promotes the activation of CO2 into CO32- species, greatly enhancing the reaction kinetics of CO2 reduction, and the OH- can also act as hole acceptor, accelerating the proton transfer from H2O oxidation. Metallic Cd has also been used as cocatalyst to couple with semiconductors for photocatalytic CO2 reduction. Inoue et al. [145] reported that the Cd decorated ZnS (Cd/ZnS) increased the reaction efficiency of photocatalytic reduction of CO 2 to
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HCOOH. c) Non-metallic (or metal free) hybrid catalyst.
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Although most efforts have been concentrated on the metal-based hybrid catalyst with superior activity, the cost is still very high, as those metals are limited in supply, and refining
process is less environmental-friendly. By stark contrast, the non-metallic nanohybrids, mainly
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composed of semiconductor-based (mainly oxides, compound semiconductors and g-C3N4),
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carbon based (e.g., GR, reduced graphene oxide (rGO), carbon nanotube (NT)) or metal-organic
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framework (MOF)-based materials show many excellent features, such as easy synthesis,
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highly designable morphologies, high surface area, good electric conductivity, nontoxic,
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inexpensive, and feasible for mass fabrication, leading to a promising material for photocatalytic CO2 reduction. For example, recently, Guo et al. [146] reported a g-C3N4@CeO2
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hollow heterostructure with rich OVs. This hybrid demonstrates a significant synergetic effect in catalysis. The OVs of g-C3N4 @CeO2 is found to play the vital role for CO2 reduction,
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resulting in the generation of CH4 much earlier with higher concentration than that upon pristine g-C3N4 and CeO2 alone. Yu et al. [96] reported a g-C3N4/ZnO structure, which exhibited
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stronger and wider light absorption than pure g-C3N4 in the visible range. The photocatalytic activity of this g-C3N4/ZnO for CO2 reduction to CH3OH is 2.3 times higher than that of pure g-C3N4. Rather than the conventional heterojunction-type mechanism, a direct Z-scheme mechanism was proposed to interpret the enhanced properties of the g-C3N4/ZnO system, where the intimate contact of the two components results in more efficient electron transfer from ZnO to gC3N4 without an electron mediator can take place at the g-C3N4/ZnO interface (Figure 6). The 19
mechanism is further confirmed by PL analyses, photocurrent measurements and DFT calculations. Kocí et al. [147] find kaolinite/TiO2 composite shows higher photoactivity than P25 for CO2 conversion to CH4, The enhanced performance is attributed to two key factors: (1) the decrease of anatase crystallite size when TiO2 NPs was introduced into the kaolinite structure;
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(2) the modification of acidobasic properties of catalyst surface inhibiting the charge
recombination and preventing the aggregation of TiO2 during reaction. Backov and Fécant [148]
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also reported novel SiO2-TiO2 self-standing porous foam with a size of millimeter length scale for 3D-photodriven CO2 catalytic reduction. The foams bear a strongly multidiffusive performance, in which photons can be greatly trapped within the cores by increasing the light
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pathway. This 3D process largely promotes selectivity for the hydrocarbon production of
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methane (~ 80%) and ethane (~ 18%). Olivo et al. [149] reported that N-doped CuO-TiO2
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photocatalyses CO2 photoreduction in gas phase under a very mild reaction condition. The study
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of N and Cu amount effect on catalytic performances showed that the CH4 formation is sensitive
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to the amount of Cu, and the reagent ratio also strongly affects the catalytic performance. Fang et al. [150] reported CuO-doped TiO2 hollow microspheres in light-driven CO2 conversion with
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H2O to CH4., and the crystal structure effect on CO2 conversion has also be investigated. Hollow spherical mesoporous TiO2 NPs possess large surface areas and layered nanostructures, with a
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large number of surface-active sites. The CuO clusters on TiO2 surface act effectively as cocatalyst to improve photocatalytic activity. Compared with the most prevailing photocatalyst,
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e.g. P25 TiO2, the TiO2 hollow microsphere catalyst hybridized with CuO exhibited higher photocatalytic activity for photo-driven reduction of CO2 with H2O into CH4. Cuprous oxide (CuxOy) nanoclusters coupled with binuclear units of ZrOCoII were covalently anchored on a silica mesopore surface to form a hierarchical assembly for CO2 reduction. The oxidation state of the Cu on surface can be tuned by the reduction of CuxOy cluster via a metal-to-metal charge transfer state of the ZrOCoII light absorber. The production yield of the HCOOH showed strong 20
dependence on the oxidation state of the surface Cu, indicating that the heterobinuclear unit functions as light absorber to donate electrons to the CuxOy catalyst to drive the CO2 reduction [151]. GR materials possess excellent conductivity and optical property, thus have been broadly applied in photocatalysis to enhance the efficiency of photo-charge separation and
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transportation. rGO modified TiO2 reveals improved activity for photoreduction of CO2 to CH4 under visible light, which is six-fold higher than the pure TiO2 [152]. Tu et al. [153] successfully
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fabricated robust hollow spheres which consist of molecular alternating Ti0.91O2 NPs and rGO nanosheets toward the photocatalytic conversion of CO2 in the presence of water vapor. The conversion of CO2 using rGO-Ti0.91O2 hollow spheres is 5-times higher than pure Ti0.91O2
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hollow spheres and 9-times higher than P25 [154,155], since i) the ultrathin nature of the
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material favors the rapid migration of charge carrier to surface, ii) the strong interaction of
incident
light
trapping
by
multi-scattering
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the
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Ti0.91O2 with rGO enhances the lifetime of charge carriers, and iii) the hollow structure increases effect.
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core-shell
structured
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rGO@CuZnO@Fe3O4 microspheres was fabricated by Jain et al [156], which is a highly efficient, recyclable and magnetically separable catalyst for CO 2 reduction under visible light
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to produce methanol. The excellent activity of this hybrid is ascribed to the restoration of the sp2 hybridized aromatic system of rGO, which promotes the movement of electrons and leads
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to efficient charge separation. The synthesized heterogenous photocatalyst could readily be collected by magnet and reused for several cycles without significant activity loss.
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MoS2-based nanomaterial has attracted wide attention as they possess a suitable band gap
for visible-light harvesting and large surface are for reactant adsorption, rendering them a promising catalyst for photocatalysis [157–159]. For example, very recently, Li and co-workers [158] synthesized a marigold-like nanoflower constructed by SiC@MoS2 nanosheets, which demonstrated excellent activity for CO2 reduction under visible light irradiation to form CH4 (Figure 7). The CH4 evolution efficiency was reported to reach the value of 323 μL·g-1·h-1 along 21
with nearly stoichiometric evolution of O2 under visible light without using any sacrificial reagents during reaction, which is a record for simultaneously visible-light driven photocatalytic CO2 reduction to CH4 and water oxidation to O2. This superior photocatalytic performance can be ascribed to (1) the Z-scheme heterostructure of the SiC@MoS2 nanosheets, which favors electron transfer upon more negative CB of SiC and more positive VB of MoS2;
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(2) the higher hole mobility of MoS2 and higher electron mobility of SiC. (3) the large surface
area of marigold flower-like morphology of SiC@MoS2; (4) the gas-solid reaction mode, which
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is beneficial for adsorption/desorption of reactants/products.
Bismuth based semiconductors, such as Bi2 WO6, BiOX (X = Cl, Br, I) and Bi2S3 are cheap materials, and show excellent photocatalytic activity due to their unique optical and structural
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features [160–168]. For example, Fu’s group [168] reported a Bi2 WO6-TiO2 binanosheet Z-
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scheme heterostructure toward high selectivity CH4 formation (Figure 8). They revealed that
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an interaction and a competition between light-driven carbonaceous residues decomposition
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and photocatalytic CO2 reduction are coexisted during reaction, suggesting the importance of
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removing organic residues from photocatalysts for an efficient CO2 reduction. Moreover, this work could provide some enlightenment on designing and/or synthesizing more efficient
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photocatalysts for CO2 reduction. BiOX photocatalysts possess unique layered crystalline structure and visible absorption property. Under visible/near-infrared-light, BiOI has been used
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as catalyst to convert CO2 into CO and CH4 [169]. Yu et al [170] developed a novel 2D/2D heterojunction of untrathin Ti3C2/Bi2 WO6 nanosheets, where Bi2 WO6 was grown along the
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surface of Ti3C2 nanosheet. This hybrid possesses a short charge transfer pathway and a large interfacial surface, leading to superior charge transfer performance. Moreover, the high surface area as well as high porosity enables Ti3C2/Bi2 WO6 hybrids enhanced CO2 adsorption capability. This 2D/2D hybrid demonstrated largely enhanced catalytic activity for photoreduction of CO2. The production yield of CH4 and CH3OH upon the hybrids is 4.6 times higher than that using pristine Bi2WO6 nanosheets. Bismuth sulfide (Bi2S3) is a p-type 22
semiconductor with a narrow band gap (1.49 eV), and the CB potential is more negative than that of most photocatalysts indicating a good visible light absorption. Recently, Bi2S3 nanoribbons with very high aspect ratio were synthesized and used for the photocatalytic reduction of CO2 to methanol [161]. Bi2S3 NPs were also used as co-catalyst to conjugate with other semiconductors, such as CeO2 [171] and CdS [172], to form nanocomposite catalyst for
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visible light CO2 reduction to CH3OH, and demonstrated dramatically enhanced activity, compared with the reactions catalyzed by each single component, which is attributed that the
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Bi2S3 co-catalyst effectively inhibits the electron-hole recombination and favors charge transfer. Since surface OVs are believed to play a decisive role in determining the efficiency of photocatalytic reactions, very recently Zhang and co-works [173] published an excellent review
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discussing about the OV-mediated photocatalysis at the surface molecular level using 2D
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BiOCl as the platform, indicating the importance of Bi based semiconductor in photocatalysis.
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Besides the inorganic materials, coordination polymers and MOF have been largely applied
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in photocatalytic CO2 reduction [174–181]. These organic metal complex materials provide a
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newly emerging platform to integrate light-harvesting and catalytic centres for photocatalysis due to their high diversity of structure and controllable synthetic procedures [150–155]. An
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artificial Z-scheme system of p-type InP/Ru complex polymer-TiO2 hybrid photocatalyst was introduced for the photoelectrochemical reduction of CO2 to formate. Upon this hybrid material,
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selective photoreduction of CO2 to HCOO− was achieved in aqueous media, in which H2O was used as both an electron donor and a proton source. The selective yield of HCOO− is more than
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70% and the conversion efficiency from solar energy to chemical energy is 0.03 ~ 0.04% [182]. Amino-functionalized NH2-UiO-66(Zr), MOF-253 supported active Ru carbonyl complex (MOF253–Ru(CO)2Cl2), amine-functionalized Ti-based porous MOF (NH2-MIL-125(Ti), (Ti8O8(OH)4(NH2-BDC)6, BDC=benzene-1,4-dicarboxylate) and other types of MOFs were synthesized and used for visible light photocatalytic CO2 reduction [40,175,183–186]. For example, Xu and co-workers [186] reported the application of a porphyrin-based MOF PCN23
222 in the photo organic synthesis. Their study showed that this MOF structure can selectively capture and further photo-reduce CO2 efficiently under visible light. The deep electron trap states present in the unique structure of PCN-222 effectively suppress the electron-hole recombination compared to the corresponding porphyrin ligands, leading to significantly enhanced photocatalytic conversion of CO2 to form formate anion. There are still many other
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types of photocatalysts in this research domain, such as Perovskite oxide and biological
catalysts, this information can be found in other excellent reports and reviews [25,27–30,187–
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190]. 2.1.3 Factors affecting products
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In general, the heterogenous photocatalytic CO2 reduction is started from the adsorption and
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activation of CO2 on catalyst surface; subsequently, PET from catalysts to the adsorbed CO2
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triggers chemical reactions which eventually determine the final products and the efficiency of
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the photocatalysis [191–193]. The process of adsorption and activation of CO2 on catalyst surface is the key step which significantly affects the following hydrogenation of CO2 and the
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suppression of competing H2 evolution reaction. Basically, CO2 adsorption induces the formation of partially charged species CO2•−, which present on catalyst surface in three modes
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[191–193]. If CO2•− is interacted with surface atoms of catalyst via O atom(s) (Figure 9 a), it
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prefers to take a H atom to form a formate anion (HCOO−) and then form formic acid (HCOOH) through protonation as a final product, where the C−O bond is stable and no cleavage occurs [194,195]. If the bonding is taken place between C atom of CO2•− (Figure 9 and c), CO2•− tends
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to combine with a H to form carboxyl radical (•COOH) and then split into the adsorbed CO, where the C−O bond breaks easily [195,196]. The adsorbed CO can be subsequently hydrogenated to form various hydrocarbons. Therefore, in principle, by the rational design of catalyst surface, the adsorption and activation of CO2 on catalyst surface may be regulated, leading to the formation of desired products [191–193]; however, a good control to the 24
adsorption process is a considerable challenge in practice and the understanding of the exact mechanism of catalysis is still limited. There are also many factors influencing the activity and selectivity of the catalysis, such as OVs, exposed facet, surface area (or porosity), heterojunctions and so on, which may show combined effects to reactions. In many cases, the results are unpredicted from the initial design. Although a clear law affecting the selectivity of
surface chemical and solution pH) are extracted from some published results.
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1) Cocatalyst effect
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the reaction is too complicate to be concluded, several important parameters (co-catalyst,
As mentioned above, today, nearly all efforts have been concentrated on development of heterostructures, among which the hybridization of semiconductor with cocatalysts dominates
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the research domain. The cocatalyst system can greatly improve the efficiency of photocatalytic
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CO2 reduction, as it can essentially improve the surface reaction kinetics by decreasing the
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activation energy. A report of the CuO loaded P25 from Gunlazuardi et al. [197] showed a
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lower activation energy than P25 (23 vs. 26 KJmol-1), indicating that the presence of CuO
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enhanced the catalytic efficiency [197]. The Schottky barrier at the interface of metal NPsemiconductor can improve the charge separation leading to an electron accumulation at the
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metal side for catalysis [198]. The adsorption of CO intermediate on catalysis, the hydrogen evolution and CO2 activation overpotentials are the key factors determining the selectivity
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[199,200]. The metals, such as Pb, possessing high hydrogen evolution and CO 2 activation overpotentials usually lead to the formation of formate as the major product; while for the
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metals, e.g. Au, with medium hydrogen evolution overpotential and weak CO interaction, CO turns to be the major product. For Pt and Fe, as they have very low hydrogen evolution overpotential and strong CO bonding ability, the major product is preferable to be H 2. An exception is Cu, which can still improve the reaction of adsorbed CO with hydrogen to form CH4 and CH3OH [42,49].
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Noble metals, e.g., Pd, Pt, Au etc., are excellent cocatalyst to improve the reaction rate and vary the selectivity. For pristine TiO2, CO was the main product with only traceable amount of CH4. By stark contrast, after loading with Pd and Pt, CH4 became the main product instead of CO [201]. Fine structure of metal NPs also shows important effect on activity and selectivity. For example, the edge sites of Au nanowires are highly beneficial for CO generation. The
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increase of edge to corner ratio of the Au atoms on NP surface led to higher production of CO.
In general, metals loaded on semiconductor surface mainly act as electron traps to promote
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electron-hole separation, and form a high local electron density area at the metal, which is favorable for the generation of CH4 [139]. 2) Surface chemical effect
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The catalysts are obtained through various synthetic methods, e.g., hydrothermal, wet-synthesis,
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physical treatment etc., leading to different surface features of the materials. Catalysis is
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fundamentally a surface reaction, which highly relies on the surface chemistry of the catalyst.
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Therefore, surface features of the catalyst, such as functional groups, play a crucial role
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affecting the CO2 reduction from the aspects of activity and selectivity. As water is involved in nearly all heterogenous CO2 reduction photocatalytic process, surface wetting or hydrophilic-
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hydrophobic property becomes a primary factor impacting on the subsequent steps. By varying hydrophilic-hydrophobic feature of photocatalyst, some reports have shown that selective
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production of either CH4 or CH3OH could be feasible. For example, the hydroxyl groups (-OH) on Ti-beta zeolites surface can be tuned by using OH− and F− anions during hydrothermal
A
synthesis to form Ti-beta (OH) and Ti-beta (F), which showed different adsorption ability to CO2 and water [202]. CO2 photoreduction under UV light over the two catalysts demonstrated quite distinct results. Hydrophilic Ti-beta (OH) demonstrated much higher reactivity than hydrophobic Ti-beta (F), whereas the selectivity of CH3OH formation from Ti-beta (F) is much higher than that from Ti-beta (OH) (41% vs. 11% in all products), indicating that increase hydrophobicity of this material favors the selective formation of CH 3OH with a reduction of 26
CH4 proportion [202]. This observation was confirmed again by introducing F into Ti/FSM-16 to increase its hydrophobicity, where the production yield of CH 3OH was increased greatly [203]. Amine groups [204] also show impact on the reaction selectivity of product formation, since they can interact with CO2 via the formation of C-N bonding, leading to the formation of to carbamate (NH2COO-) and subsequent carbonate. This process resulted in the generation of
IP T
carboxyl radicals and then decomposed into CO on catalyst surface as the main product.
SC R
3) Solution pH effect
CO2 solubility in water can be changed by varying pH value in solution, which also affects the concentration of CO32− and HCO3− in solution. The different chemical species generated by
U
dissolved CO2 lead to different adsorption modes and reactions on catalyst surface. It is also
N
found that the pH can affect the completing hydrogen evolution during CO2 reduction. Reaction
A
is preferable to produce H2 at low pH [42]. An early study [205] of electrochemical CO 2
M
reduction test using Cu electrode and various electrolyte solutions showed that the adsorption strength of CO on Cu is much weaker than that on Pt. Adsorbed CO is reduced to CH4, C2H4,
ED
EtOH and isopropanol at more negative potentials. The product types depend strongly on the
PT
electrolytes employed. Formation of C2H4 and alcohols is favored in solution of KCl, K2SO4, KClO4 and dilute HCO3– with pH ~6-7, whereas CH4 is the major product in relatively
CC E
concentrated HCO3– (pH = 9.6) and phosphate solutions (pH = ~6). The product selectivity depends on protons on electrode surface, which is controlled by pH or the electrolyte, since OH− is released in the electrode reactions. However, whether this observation in
A
electrochemical catalysis can be applied in heterogenous photocatalysis is still unknown. The pH of the reaction solution may play an important role in the preferred surface reaction, leading to different final products. 2.2 Photocatalytic Conversion of Organics
27
In chemical industry, the synthesis of many important organic chemicals usually requires harsh operating conditions, such as high temperature and pressure, with extreme amount of energy consumption. Heterogenous photocatalytic organic synthesis is a concept to use light as an energy source and small hydrocarbons as the raw material (reactant) to produce value-added organics, which indicates a more promising route to drive chemical reactions. Moreover, many
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chemicals synthesized upon photocatalytic process can be achieved in simple “one-pot” reactions. The following content briefly reviews the state-of-the-art progresses of the
SC R
heterogenous photocatalytic organic conversions. Depending on the reactions, this section is
mainly divided into four parts, including the oxidation of alcohol and aromatics, carbon-carbon
U
coupling, esterification and epoxidation, nitrogen containing chemical formation.
N
2.2.1 Oxidation of alcohols and aromatics
A
In heterogenous photocatalytic water splitting, alcohols (e.g. CH3OH) have been mainly employed
M
as hole scavengers to promote the reaction, because the redox potentials of alcohols are less positive than the VB edge of the semiconductor catalysts [206]. In principle, alcohols can be
ED
photocatalytically oxidized to the corresponding carbonyl compounds [207–214]. However, due to
PT
the difficulty of the reaction control, this reaction usually produces carboxylic acids or even CO2, which are products of over-oxidizing or fully oxidizing (decomposed) processes. Therefore, the
CC E
control of oxidizing extent and the selectivity toward desired products is the primary concern for heterogenous photocatalytic oxidation of organics. Studies have shown that the selectivity can be influenced by many parameters, such as the medium (solvent), the photocatalyst itself, the presence
A
of O2, and the type (structure) of the alcohol used. Another similar photocatalytic oxidation process is the oxidation of aromatics, which leads
to the formation of many important chemicals, such as resins and pharmaceutical products, where phenol, catechol and hydroquinone have been widely used as precursors in this reaction. The heterogenous photocatalytic oxidation of aromatic compounds was firstly reported by Fujihira et 28
al. [215], where the role of various reaction parameters, such as oxygen, solution pH, and additional of ions, have been investigated in the selectivity of the hydroxylated products. Friend et al [216] studied the formation of methyl formats by oxidizing methanol on TiO2 (110) surface. They found that exposure of the TiO2 surface to O2 is important to promote the photo-oxidation reaction in order to heal surface and near-surface defects that can serve as hole
IP T
traps. Based on measurement using mass spectrometry and scanning tunneling microscopy, two
consecutive photo-oxidation steps for this reaction were proposed. The first step is to form
SC R
CH3O and water via thermal dissociation of the O–H bond, followed by the formation of formaldehyde through hole-mediated oxidation of CH3O. In the second step, transient HCO is
produced from formaldehyde, which can react with residual CH3O to produce methyl formate
U
as the final product. CeO2, as a reducible metal oxide, can replace TiO2 as the supporting
N
material of Au NPs, which may endow different catalytic properties. Kominami et al. [217]
A
prepared Au/CeO2 plasmonic photocatalyst by reducing H4 AuCl4 on CeO2 surface with citric
M
acid as reducing agent. Upon 530 nm-wavelength excitation, Au/CeO2 can catalyze the
ED
selective oxidation of aromatic alcohol to aromatic aldehydes with O2 in aqueous phase, and the reaction is almost quantitative. In addition, the catalytic activity of the catalyst mainly
PT
depends on the specific surface area of Au NPs loaded on CeO2, rather than the total amount of Au NPs. The sample with Au NPs of 30 nm demonstrated the most efficient catalytic
CC E
performance. Compared with the single-step synthesis, the multi-step method allows the formation of the Au NPs with larger particle size and higher catalytic activity, and their reaction
A
rate is twice higher than those NPs obtained using the single-step synthesis method. The oxidation of alcohols over the Au/CeO2 composites proceeds with high selectivity, e.g. the photocatalytic system endows the oxidation of p-aminobenzyl alcohol to p-aminobenzaldehyde at high product yield (99 %) without exerting any side effect to amino groups (Figure 10) [218,219]. Due to the SPR effect of Au under visible light irradiation, hot electrons are injected to the CB of adjacent CeO2, resulting in the electron-deficient Au. The efficient oxidation of 29
benzyl alcohol may be attributed to smooth electron transfer from electron-rich benzyl alcohol to the electron-deficient Au. Because in this reaction O2 is involved, electrons trapped by O2 may form highly active oxygen species, such as O2•− radical and/or H2O2. In the presence of metal co-catalyst, one-electron reduction of O2 takes place more efficiently in photocatalytic oxidations than the multielectron reduction of O2. The high selectivity of benzaldehyde implies
IP T
that those active oxygen species may be exhausted in the reaction system., The Au based plasmonic catalyst can also catalyze hydrogen peroxide to oxidize aromatic alcohols to
SC R
corresponding carbonyl compounds [220] with a conversion rate close to 95%, under visible light irradiation. Li et al. [221] also presented a plasmonic Au/CeO2 hybrid nanofiber catalyst for selective oxidation of benzyl alcohol to benzaldehyde with O2 under visible light irradiation.
U
The Au loading effect on catalysis was studied and showed that an optimal level over 0.5 wt. %
N
Au-loaded CeO2 nanofibers led to the highest selectivity of 100 % benzaldehyde formation.
A
Shiraishi et al. [222] prepared Au/TiO2 plasmonic photocatalyst by loading Au NPs smaller
M
than 5 nm on the interface of anatase/rutile TiO2 mixed crystal structure (Figure 11). At room
ED
temperature, aromatic alcohol can be efficiently converted to oxo-compound with high catalytic selectivity using Au/TiO2 as the catalyst with visible light irradiation.
PT
Hollmann et al. [223] have developed a photocatalytic technique, combining
CC E
photochemical water-oxidation catalysis with peroxygenase catalysis to achieve visible-lightdriven, aerobic oxidation of hydrocarbons with high selectivity. The generation of H2O2 during photocatalytic
water oxidation
process
promoted
peroxygenase-catalysed,
selective
A
oxyfunctionalization reactions. This concept was testified for the reaction of the selective photoenzymatic hydroxylation of ethyl benzene using Au-TiO2 photocatalyst in the presence of Agrocybe aegerita (AaeUPO) under visible light irradiation (Figure 12-a). More importantly, a broad range of aliphatic and aromatic compounds have also been converted into their corresponding alcohols, verifying the versatility and robust of this technique. Immobilized Au 30
NPs containing layered titanate in interlaminar space as catalyst promoted the oxidation of benzene in the presence of water to the corresponding phenol with high product yield and selectivity (96%) under visible light irradiation [224]. Huang and co-workers [225] prepared a series of plasmonic catalyst M@TiO2 (M=Au, Pt, Ag) using a simple in-situ growth approach to load noble metals onto TiO2 (Figure 12-b). Among the three catalysts synthesized, Au/TiO2
IP T
demonstrated the highest catalytic activity for visible-light hydroxylation of benzene in the aqueous phase. The selectivity of the reaction can reach 91% with the phenol yield of 63%.
SC R
Tada et al. [226] found that the oxidation of alcohols into carbonyl compounds in aqueous phase could be effectively catalyzed by the addition of trimethyl octadecyl ammonium chloride
U
(C18TAC) in the system of Au/TiO2 under visible light irradiation (Figure 13) with high
N
selectivity (>99%). Compared with the reaction without surfactant, the addition of cationic
A
surfactant C18TAC to the photocatalytic system with an appropriate concentration leads to the
M
increase of the production yield with a factor of 3.3~5.7 due to the formation of stable micelles as nanoreactors which favor the surface reactions at the strictly confined region. As a result, for
ED
some individual alcohol reactants, the yield can be increased by 29.6 times.
PT
Very recently, Li and co-workers [227] reported an application of Bi2 WO6-x/amorphous BiOCl (p-BWO) nanosheets for photocatalytic activation of benzylic C-H bonds in toluene via
CC E
a selective photocatalytic oxidation process. The p-BWO nanosheets show distinct appearance of blue color from pristine Bi2WO6, with the irradiation of visible light and the presence of atmospheric O2, as shown in Figure 14 a and b. Structural characterization confirmed that
A
abundant W(VI)O6–x units are presented in the hybrid, which serve as the active sites to rapidly capture and consume the photogenerated electrons, thus effectively favoring the separation of electron–hole pairs. Due to this unique advantage, p-BWO nanosheets could effectively activate C(sp3)–H bonds in photocatalytic oxidation of toluene, which is a challengeable and significant reaction in chemical synthesis. This catalyst demonstrated a remarkable enhancement in 31
performance for the photocatalytic oxidation of toluene to produce benzaldehyde and benzoic acid with high selectivity. The highest conversion rate reached 2162μmol g−1h−1, which is 166 times higher than that using pristine Bi2WO6 nanosheet (Figure 14 c and d). Alloy NPs of noble metals with strong visible light absorption are capable of effectively increasing the conversion of several reactions, including Suzuki-Miyaura cross-coupling,
IP T
oxidation addition of benzyl amine, selective oxidation of aromatic alcohols, and phenol
oxidation. The reaction of oxygen oxidation of benzyl alcohol catalyzed by Au-Pd/ZrO2
SC R
[228,229] or Au-Cu/TiO2 [230] can be further improved by visible or solar irradiation. For AuPd/ZrO2 system, the distribution of Pd active sites and the heterogeneity of electrons around the
U
NP surface are determined by the molar ratio of Au-Pd, playing a critical role in catalysis.
N
Further study reveals that Au-Pd alloy can also drive the dissociation of α-H from aromatic
A
alcohols under visible light irradiation, favoring the oxidation reaction in a mild condition
M
instead of high temperature and highly concentrated oxygen (Figure 15) [229]. By introducing a small amount of metal ions (Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and
ED
Zn2+) in to CdS (see Figure 16), the mobility of photogenerated carriers at the interface between GR and CdS semiconductor can be largely enhanced [231]. This simple design can significantly
PT
improve the efficiency of selective photo-oxidation-reduction reactions under visible light,
CC E
including aerobic oxidation of alcohols and anaerobic reduction of nitro compounds. It also favors the suppression the recombination of electron-hole pairs, while retards the negative "shielding effect" caused by the high concentration of GR. Under the same reaction conditions
A
in the presence of CdS-GR (10 wt %) catalyst, the conversion of benzyl alcohol to benzaldehyde after 2 hours irradiation was approximately twice higher than that obtained using CdS with 5 wt % GR as catalyst [231,232]. A series of CdS-GR nanocomposites with different weight addition ratios of GR was also synthesized by a simple one-step solvothermal method. CdS-GR nanocomposites have been shown to be of high efficiency for selective aerobic oxidation of 32
alcohols and reduction of heavy ion Cr(VI) rather than water. In addition, the photocatalytic activity of CdS-GR nanocomposites can be effectively improved by using solvent stripped GR (SEG) instead of GO as a precursor of GR, which can effectively convert benzyl alcohol to benzaldehyde in water [231–233]. The ternary nanocomposite has been reported to be a more efficient photocatalyst compared to CdS-GR for the selective oxidation of benzyl alcohol to
IP T
benzaldehyde [234].
Oxidation of CH4 to methanol with high selectivity has recently been reported by Tang et
SC R
al. [235] using a Fe species decorated TiO2 photocatalyst at ambient conditions. The catalyst
with an optimal composition demonstrated a 15% conversion rate for CH4 with an alcohol
U
selectivity of more than 97%, where the methanol selectivity is over 90%. The production of
N
18 moles of alcohol per mole of iron active site has been achieved in 3 h. Various
A
characterization technique confirm the function of the major iron-containing species—namely,
M
FeOOH and Fe2O3, which enhance charge transfer and separation, decrease the overpotential of the reduction reaction and improve selectivity towards methanol over carbon dioxide
ED
production. The mechanism of this catalysis was proposed that upon photon absorption, electron / hole pairs are generated on TiO2 surface. As the CB potential of Fe2O3 is lower than
PT
that of TiO2, photogenerated electrons can be then transferred to the CB of Fe2O3, leading to
CC E
the reduction of H2O2 to form·-OH radicals. Simultaneously, CH4 can react with photogenerated holes at the VB of TiO2 to form methyl radicals (CH3), which then react with the -OH radicals to form methanol on Fe species.
A
To increase the sustainability and production of biorefinery, intensive effort has been
focused on the efficient utilization of the three components of lignocellulose from biomass, in which lignin is believed to be the most abundant source of renewable aromatics. However, challenge still exists in the direct transformation of native lignin from biomass. For example, conventional approaches, such as Kraft and Organosolv processes, usually involve the 33
fractionation of lignocellulose, which can cause transformation of lignin structure. In particular, the condensation to form more C-C linkages by consuming β-O-4 bonds, inducing the inactivation of the extracted lignin [236]. Very recently, a more advanced achievement of the heterogenous photocatalytic selective organic oxidation has been reported. The concept is based on the selective fractionation / oxidation of cheap biomass (macromolecules or polymer
IP T
composite) to form valuable organic products. A report from Wang et al. demonstrates a lignin-
first strategy based on solar energy-driven β-O-4 ether bond cleavage under mild conditions
SC R
using CdS quantum dots (QDs) as the photocatalyst (see Figure 17) [237]. The distinct
advantage of CdS QDs is their high selectivity in this reaction without segmenting other bonds, thus the functional groups of lignin compounds can be secured. More importantly, upon the
U
photocatalytic process using CdS QDs, native lignin compounds in birch woodmeal can be
N
directly converted into functionalized monomeric aromatics with a yield of 27 wt%, which
A
reaches 84 % of the theoretical maximum calculated from the cleavage of the β-O-4 linkages.
M
A new electron-hole coupled (EHCO) mechanism has been proposed for the precisely cleaving
ED
the C−O (β-O-4). In principle, the Cα−H bond is first fragmented upon oxidation by holes, forming Cα radical intermediate, which greatly decreases the bond dissociation energy (BDE)
PT
of the β-O-4 bond. Therefore, upon the photo-excitation of CdS QDs, β-O-4 bond breaking is promoted by the Cα radicals. The remarkably reduced BDE of β-O-4 bond in the Cα radical
CC E
intermediate is the dominating factor, leading to the high activity and selectivity of the lignin transformation. After reaction, CdS QDs can be facilely separated by forming aggregates upon
A
an addition of acetone into the liquid. 2.2.2 Carbon-Carbon bond formation (C-C coupling) Kim et al. [238] reported a hybrid structure composed of Pd NPs on exfoliated 2H-WS2 nanosheets (WS2/Pd). This catalyst demonstrated extraordinarily high photocatalytic activity in Suzuki reactions for C-C coupling under visible light (Figure 18). The photocatalytic activity 34
of the WS2/Pd showed size-dependence on Pd NPs. The increase of Pd NP size on WS2/Pd hybrid led to the enhancement of photocatalytic activity in Suzuki reactions at room temperature. Moreover, the protic organic solvents also affected the catalysis, which can activate WS2/PdN catalysts in photocatalyzed Suzuki reactions; whereas they had been considered less active compared with the polar aprotic ones in conventional Suzuki reactions.
IP T
A plausible mechanism suggested that photogenerated holes can be transferred to the protic
organic solvents, while the photogenerated electrons are transferred to Pd NPs, which favor the
SC R
efficient electron / hole separation. This charge transfer process endows the electronenrichment on Pt NPs and accelerates the oxidative addition of aryl halides reactions. Xie et al. [239] report the visible-light-driven dehydrogenative coupling of methanol into ethylene glycol,
U
using CdS NPs (TEM images in Figure 19) as the heterogenous catalyst and methanol as the
N
raw material under a mild condition. Conversion ratio of ethylene glycol over CdS nanorod
A
catalyst modified by MoS2 nano-foam reached 90% with high selectivity. Mechanism study
M
indicates that the activation of C−H bond rather than O−H bond in CH3OH by photogenerated
ED
holes on CdS through a concerted PET (CPET) process results in the generation of the hydroxymethyl (⋅CH2OH) intermediate on CdS surface, and the weak adsorption of CH3OH
PT
and ⋅CH2OH radical enables them to be readily desorbed from CdS surfaces for subsequent CC coupling to form EG. Additionally, the MoS2 nano-foam with rich edge position is highly
CC E
beneficial to the reaction efficiency. The reaction selectivity of EG has also been increased by the enrichment of mesoporous MoS2 nano-foam.
A
Wang et al. [162] reported a unique truncated tetragonal bipyramidal bismuth vanadate
(BiVO4, BVO) single crystal structure, with tuneable {010} and {110} facets, for photocatalytic C-C coupling of formaldehyde into EG under visible light. Chloride anions was used to control the growth of the single crystals as well as the ratio of exposed {010} and {110} facets. The BVO with the facet ratio of 1 : 1 allows the highest conversion yield of C2 compound. Pt@MoO and MoO 35
NPs were loaded to the two facets of the BVO, respectively, which further enhanced the conversion ratio (Figure 20). This greatly increased property is attributed to the unique architecture, where photo-charge separation can be largely accelerated by the Pt and MnOx NPs, while the HCHO coupling may be catalyzed at the MoOx shell via a redox process. 2.2.3 Esterification and epoxidation
IP T
Using alkyl alcohol as the reactant, ester has been synthesized via a heterogenous photocatalytic
approach [240]. With H14[NaP5W30O110] loaded silica as catalyst, Fatemeh and co-workers
SC R
successfully demonstrated the esterification of salicylic acid with fatty alcohol and benzyl
alcohol. When the reaction time is 3 h and the starting molar ratio of acid to alcohol is 1:5, the
U
yield of ester reaches 85%.
N
Zhu and Sarina [241] reported the photocatalyzed selective epoxidation of alkenes using
A
TiN supported Cu NPs (Cu@TiN) as the photocatalyst (Figure 21). During photocatalysis, a
M
chemisorb of reactant alkenes on Cu NPs takes place and form Cu-alkene complexes at catalyst surface, which can be activated by photon absorption. The cyclic ether solvent also plays a
ED
crucial role, which can facilitate the reaction by reacting with the adsorbed O2 on Cu NPs under irradiation to produce oxygen adatoms. The activated surface Cu-alkene complexes can then
PT
react with the adatoms to form corresponding epoxides. This photocatalyst exhibits excellent
CC E
activity and selectivity with various types of alkenes for photocatalytic epoxidation. Irradiation wavelength and intensity showed important effect on the epoxidation, indicating that photogenerated electrons of Cu NPs drive the reaction.
A
Plasmonic photocatalyst has also been applied for visible-light-driven selective
epoxidation of organics [242]. Ethylene can be selectively oxidized to ethylene oxide catalyzed by Ag cubes supported on Al2O3 with the side length of 60 nm under visible light. At 450 K, the oxidation rate under visible light irradiation is 4 times higher than that in the conventional heat-driven system. The increase of reaction rate is dependent on the temperature, which varies 36
from 8 times at low temperature to 3 times at high temperature, without affecting the selectivity in reaction [243]. Under ambient conditions with visible light, the ZnS-GR nanocomposite exhibited photocatalytic activity for aerobic selective oxidation of alcohol and epoxidation of alkenes. The GR in ZnS-GR works as an organic dye-like macromolecule "photosensitizer" for ZnS rather than an electron reservoir. Cu can also be used as a catalyst for some reactions, but
IP T
it is rarely used to catalyze epoxidation of alkene as it can be easily oxidized. Linic et al. [244] used Cu NPs with an average size of 41±9 nm attached on SiO2 (Cu/SiO2) for catalyzing the
SC R
epoxidation of propylene. Under visible light irradiation, the selectivity of the reaction is greatly
enhanced. The reason is that the localized surface SPR effect of CuNPs weakens the Cu-O bond, thus promoting the reduction of Cu2O to Cu0 and the consequent electron transport.
U
2.2.4 Carbon-Nitrogen bond transform
N
Photocatalytic N-alkylation of ammonia in alcohol to amines has been studied by Ohtani
A
et al. in an early report, using Pt-TiO2 as photocatalyst, where Pt plays an important role of the
M
selectivity of the reaction [245]. Secondary amines have also been synthesized using primary
ED
amines in alcohols aqueous solution (Figure 22) [245]. Pt decorated TiO2 and CdS catalysts showed good activity to synthesize amino acids (e.g. glycine, alanine, serine, aspartic acid, and
PT
glutamic acid) in an aqueous solution of ammonia and methane [246–248].
CC E
Recently, Stíbal et al. [249] reported the photocatalytic N-alkylation of aniline with AuTiO2 as catalyst and primary alcohols as reactant (Figure 23). The overall selectivity of the mono alkylation and two alkylation products is always higher than 70%, and depends on the
A
alkyl alcohol used. It is proposed that the reaction is triggered by the oxidation of the primary alcohol to the corresponding aldehyde through the R-O• or OH• radical intermediates by the photo-generated holes of TiO2. The photo-generated electrons transferred to the Au surface reduce the resultant protons to molecular hydrogen, which subsequently react with the aldehyde to accomplish the alkylation. TiO2/maghemite-silica nanocomposite as a photocatalyst has also 37
been reported for the selective oxidation of benzyl alcohol to benzaldehyde. When the conversion of benzyl alcohol is about 50%, the unprecedented selectivity of benzaldehyde in organic medium reaches 90% [250]. Nitroaromatic compound (e.g. 4-nitrophenol (4-NP)) can be reduce onto corresponding aminoaromatic (e.g. 4-aminophenol (4-AP)) in the presence of Au or Pt catalyst with NaBH4
IP T
as the reducing agent [251–253]. A recent study from Ma’s group [254] showed that the reaction rate of this reduction process can be further increased under photo-irradiation using a Au-TiO2
SC R
catalyst. As illustrated in Figure 24, it is proposed that under UV light, Au works as electron sink to accept the photo excited electrons from the CB of TiO 2. The electrons are then injected
U
into the LUMO of the 4-NP for reduction. Meanwhile, the high electron density on Au favors
N
the formation of hydrogen or Au-H species on Au surface for more efficient hydrogenation to
A
4-NP. On the other hand, under visible light, a hot-electron mechanism plays dominant role for
M
reaction rate enhancement. Due to the SPR effect of Au, hot electrons were injected to CB of TiO2, which can be further transferred to the LUMO of 4-NP for reduction to 4-AP.
ED
Nitrobenzene can be transferred to imines upon the photocatalytic process using TiO2 as the
PT
photocatalyst and alcohol as dispersant (Figure 25). This reaction showed high selectivity for the cases using C1–C3 alcohols [255]. It has shown that surface properties of the TiO2 exerts important
CC E
effect on selectivity [256]. Anatase structure with a much higher Lewis acidity than rutile one can favor imine formation, while Rutile exhibited a higher selectivity for the formation of aromatic amino compound. Pt/TiO2 [257] and Au/CeO2 [258,259] are able to promote photocatalytic
A
azobenzene reduction using alcohol as a sacrificial electron and proton donor. Very recently, Shiraishi et al. [260] reported that the photocatlytic hydrogenation of azobenzene to hydrazobenzene on CdS NPs under visible light irradiation reached the selectivity more than 95%. Homoallyl amines can be obtained using olefins and trisubstituted imines as reactants and CdS based materials as photocatalyst. Shiraishi et al. [261] showed the study of photocatalytic cyclisation 38
reactions, where benzimidazoles were synthesized using 1,2-diaminobenzene in the presence of Pt– TiO2 photocatalysts in alcohol solution. Using nitroaromatic compounds in alcohols as the starting materials, Hakki et al. [262] reported the photocatalytic formation of polyalkylated quinolines over TiO2 decorated with acid modified mesoporous SiO2 catalyst (Figure 26). In this reaction, nitro compound reduction and the alcohol oxidation are proceeded simultaneously upon the
IP T
photogenerated electrons and holes, respectively. An imine is then produced upon condensation of
the generated aldehyde and amino compounds. The surface attached arene–SO3H acid group
SC R
strongly affected the selectivity of the products and production yield of the quinolines.
Morpholine derivatives are important substance for pharmacological study. Plasmonic photocatalysts can be used to catalyze the oxidative condensation of aldehydes and morpholine
U
under visible light irradiation. Luque group [263] found that in the presence of H2O2 and strong
N
alkali KOH, under 532 nm laser irradiation with Au/SiO2 as photocatalyst, amides can be
A
synthesized by the reaction of morpholine with benzaldehyde. MOFs have also been used for
M
imine formation from amines. Li et al. [185] reported a visible light assisted aerobic
ED
photocatalytic oxidation of amine to imines over NH2-NIL-125(Ti), where the photo-generated Ti3+ and O2•− formed through the reaction between Ti3+ and O2 promoted this organic
PT
transformation.
Unsupported Ru NPs have also been used as photocatalyst to direct aminate octanol and
CC E
other alcohols into primary amines in the presence of ammonia (Figure 27) [264]. Octylamine was obtained at a lower reaction rate using small Ru NPs (~ 2 nm) with the selectivity of ~ 90%
A
and conversion ratio of 92%; whereas for larger Ru NPs and supported NPs, a lower selectivity was observed for the octylamine formation with a conversion decreased to 70–80%. The primary reaction of alcohol amination into octylamine showed nearly structure-insensitive reaction, as all Ru NPs presented comparable turnover frequency values. The self-coupling of octylamine was sensitive to the NP size. Amine self-coupling is inhibited upon small NPs,
39
which can be attributed to the structural and electronic effects during hydrogenation of the secondary imine. 2.2.5 Factors affecting products Although value-added organics can be “one-pot” synthesized using heterogenous
IP T
photocatalytic process from simpler staring materials, which bears the distinct advantages of mild reaction conditions, less energy consumption and environmental-friendly, the challenges
SC R
exist in promoting reaction selectivity and reaction rate. The key issue in photocatalytic oxidation reactions is to minimize the mineralisation of products and reactants to achieve good selectivity, thus a suitable catalyst as well as optimal reaction conditions need to be systematically investigated. As
U
aforementioned, there are many impressing factors, such as solvent, catalyst structure and its
N
surface functionality etc., which may present together as combined effect during reaction. We
A
here briefly summarize several possible factors affecting the reaction efficiency and selectivity.
M
1) Effect of solvent
In reaction, solvent plays the role to disperse and transport reactants to the reaction sites of the
ED
catalyst, and also may join the reaction, therefore, solvent features, such as structure and polarity,
PT
strongly influences the interaction between the surface of the photocatalyst and the chemicals. By simply varying the solvent, photocatalytic reaction rate can be tuned. A report from Brezova et al.
CC E
[265] showed that the photocatalytic reduction rates of nitroaromatic compounds in different solvents follow the order as: methanol > ethanol > iso-propanol, because the formation of αhydroxyalkyl radicals in methanol is more efficient than that in other solvents [266]. Solvents with
A
different properties, such as diffusibility and polarity possess different ability to transport all species during reaction, including reactants, highly active intermediates and products, which exert important impact on electron transfer kinetics, subsequent reaction rates and selectivity [267]. Almquist et al. [268] reported the solvent effect using the reaction of TiO2 catalyzed oxidation of cyclohexane in various solvents including acetone, isopropanol, dichloromethane, chloroform, carbon tetrachloride, 40
benzene and n-hexane. It was found that selectivity to cyclohexanol, cyclohexanone and reaction rates were influenced by the type of solvent. Non-polar solvents (e.g. hexane and benzene) led to low selectivity with cyclohexanol being completely oxidized to water and CO2, due to their strong adsorption on catalyst surface. In polar solvents, adsorption of cyclohexanol to the TiO2 NPs is weaker as the competition for adsorption sites with the solvent, and thus the selectivity is higher.
IP T
Solvent induced competitive adsorption, to an extent, determines the overall reaction rate and
selectivity to cyclohexanone and cyclohexanol. Dichloromethane allowed the highest reaction
SC R
rate; by contrast, chloroform and isopropanol dramatically inhibited the desired reactions. The
optimal solvent for this reaction with high selectivity is the one that can minimize product adsorption on catalyst and improve reactant adsorption on catalyst surface. Colmenares et al.
U
[250] also reported the similar phenomenon, where photocatalytic oxidation of benzyl alcohol to
N
benzylaldehyde in acetonitrile demonstrated better selectivity than that in aqueous medium. Soana
A
et al. [269] reported the effect of an organic solvent to slow down the reaction and improve selectivity.
M
Water is a good “green” solvent for heterogenous photocatalytic reactions, it is of high polarity
ED
and can easily interact with the surface of the photocatalysts to react with photo-generated holes to form strong oxidizing radical of –OH•, thus resulting in a difficult control to the oxidation extent and
PT
hence inducing a poor selectivity. Therefore, polar organic solvents (e.g. the alcohols introduced above, dimethyl carbonate [270] and acetonitrile) or their mixtures are most often employed as
CC E
dispersant for photocatalytic organic synthesis. For instance, acetonitrile has been used as solvent in many photocatalytic organic syntheses, including oxidation of liquid cyclohexane [271], benzyl
A
alcohol [250], naphthalene [272], primary aliphatic amines [273], and epoxidation of propylene with high selectivity [244], as it is a stable solvent which usually cannot be oxidized by the photogenerated holes. The pH value of the solution also affects the dissociation of the reactant as well as their adsorption on catalyst surface. For example, Zhu et al. [274] reported an efficient photocatalysis to selectively oxidize 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under mild conditions using O2 in air as the oxidant and g-C3N4 supported cobalt 41
thioporphyrazine (CoPz) as photocatalyst. This reaction showed pH dependence of the solution, where the FDCA can be achieved with a yield of 96.1% in an aqueous solution at pH = 9.18, while at lower pH (4.01), the main product is 2,5-diformylfuran. 2) Effect of catalyst surface feature In heterogenous photocatalytic organic synthesis, reaction is proceeded following two major
IP T
steps [12,13,24]. The 1st step is the adsorption/dissociate process, where reactants adsorbed on catalyst surface; the 2nd step is the desorption process, where products leave the catalyst surface.
SC R
Therefore, strong adsorption between organics and catalysts (mainly semiconductors) usually
induce high reaction rate, however it may lead to the mineralisation with CO2 and water as the final products. A rational design to the catalyst surface, such as specific functional groups, can
U
result in a suitable and preferential adsorption, and thus increases the selectivity of the reaction.
N
Hakki et al. [262] reported that the formation of polyalkylated quinolines via a cyclization using
A
nitroaromatic compounds as reactants reaches a selectivity up to 53% catalyzed by arene–SO3H acid
M
group functionalized mesoporous SiO2-TiO2 hybrid, whereas this value is only 6% for pure TiO2,
ED
and 18% for SiO2-TiO2. Notestein et al. [275] designed a novel catalyst with nanocavities constructed by depositing a shape-selective sieving layer of Al2O3 on an oxide (TiO2) catalyst
PT
surface, which bears high selectivity for both reduction and oxidation reactions as shown in Figure 28. Nitrobenzene and benzyl alcohol can be photocatalytically reduced and oxidized into
CC E
their corresponding products with selectivity up to 90%, respectively, while no reaction was taken place for the ortho methylated derivatives. The excellent selectivity is resulted from the
A
size-sieving effect induced adsorption alignment of reactants to the catalyst surface. Ohtani et al. [276] found the selectivity of photocatalytic synthesis of pyridine 3-carboxylic acid (PCA) from diamines using CdS is higher than that using TiO2 as photocatalyst. High purity racemic PCA product was obtained in the presence of CdS, whereas for the case using TiO2, PCA was produced with an excess of the L-isomer as by-product. The selectivity difference was ascribed to the
42
adsorption difference of the amino group in l-Lys on catalyst surface, which underwent distinct oxidation reactions [277]. 3) Effect of incident light wavelength The efficiency of charge carrier’s generation on photocatalyst is of high importance for photocatalysis, which depends on the energy levels of the semiconductor and the energy of the
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incident light. By varying the wavelength of the incident light, the process for photogeneration of electron-hole pairs may be tuned to favor the selectivity and reaction rate for the organic
SC R
transformations.
For example, as shown in Figure 29, under visible light irradiation (λ> 450 nm), Pt NP decorated P25 (Pt/P25 catalyst) favored selective aerobic oxidation of aniline to form nitrosobenzene as the
U
main product (selectivity: 90%) [278]. By contrast, under UV irradiation azobenzene turned be the
N
major product. It is proposed that upon visible light absorption, an inter-band electronic excitation
A
of Pt atoms takes place, which promotes the electrons to be transferred to the CB of TiO2, followed
M
by the formation of O2˙− radicals on Pt surface. The O2˙− radicals are highly active to attract the H
ED
atoms from aniline upon its dissociation on Pt NPs, producing hydroperoxide species. The following reaction between anilino anion and hydroperoxide species results in the formation of nitrosobenzene.
PT
Pt/TiO2 photocatalyst has also been used to catalyze aromatic ring hydroxylation of benzene derivatives when illuminated with light of appropriate wavelength, in the presence of water as
CC E
oxidant [279]. It was observed that under irradiation with 365 ± 20 nm light, the selectivity of phenols, produced via photocatalytic hydroxylation of toluene or p-xylene was low; however, the yield and
A
selectivity was significantly increased under 405 ± 20 nm light irradiation (Figure 30). This wavelength dependant selectivity is attributed to that upon incident light at ~ 405 nm photogenerated charge carriers mainly form at the surface of the catalyst, which are more efficient to participate the hydroxylation reaction than those generated from the deeper area of the catalyst excited under ~365 nm light. The lower selectivity at 365 nm was resulted from the retardant of charge separation due
43
to the electron transfer from toluene to TiO2 upon the absorption of toluene at ~ 360 nm, leading to the generation of benzyl radicals to form dibenzyl. 3. Photoresponsive NP Initiated Radical Polymerization Apart from the photocatalytic synthesis of small organic molecules, we here introduce the advances of heterogenous photocatalytic synthesis of polymers or macromolecules from their
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monomers, to demonstrate the fundamental achievements including the reaction mechanisms
and technical advantages. Compared with CO2 reduction and organic transform introduced
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above, this domain is relatively less developed, because it needs more background from polymer chemistry and specific characterization technique of macromolecules, including
U
structure and mechanics analysis, differing from those for small molecules. We choose the free
N
radical polymerization system, as it is the most classic and major topic in polymer research.
A
3.1 Free Radical Polymerization
M
3.1.1 Simple semiconductor system
The use of semiconductors to photo-initiate polymerizations were first attempted by Markham
ED
and Oster et al. [280,281]. Both reported that ZnO NPs can initiate the free radical
PT
polymerization of methyl methacrylate (MMA) under UV ( = 365 nm) irradiation. They also demonstrated that the presence of O2 adsorbed on ZnO surface is crucial for the reaction, as no
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catalytic activity could be observed when using ZnO NPs undergone a thermal treatment, where the surface adsorbed O2 was removed upon heating. However, excess O2 in solution can inhibit the polymerization, as expected for any radical polymerization. Solvent effect study showed
A
that solvents of higher dielectric constants with good proton-donating or hydrogen-bonding characteristics favor the initiation reaction via the intervention of photo-excited surface O2. The anionic nature of the excited surface oxygen was evidenced by the fact that the polymerization can proceed readily for electrophilic monomers such as MMA and acrylonitrile, but not for styrene [280]. It was thus proposed that photo-activated oxygen was formed on the surface of 44
ZnO during irradiation and converted into radical anion (such as O 2 •-) which could then initiate the polymerization [280,281]. The photo-generation of active species on the semiconductor surface and the following initiating step of photo-radical polymerization were further investigated using quantum sized TiO2, ZnO and CdS by Hoffman and co-workers [256,282]. The effect of solvent, monomer
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concentration, NP concentration, light intensity and NP size on polymerization rate were examined. The quantum sized semiconductors exhibited significantly higher quantum yields
SC R
than their bulk counterparts. Figure 31-a presents the proposed reaction mechanism consisting of the surface reduction of the monomer by the photoexcited electrons present at CB, leading to a radical anion which can initiate the free radical propagation. An alkyl alcohol solvent (such
U
as isopropanol) must also be present in the system. It acts as hole scavenger in order to suppress
N
charge recombination, and the resulting oxidation product can also initiate the polymerization,
A
either directly or via monomer oxidation. The implication of a radical anion as pivotal initiating
M
species was confirmed by the inhibition of the reaction by O2 or acetonitrile, both of which are
ED
not only electron scavengers, but also radical anion scavengers. For the case of the MMA polymerization triggered by ZnO NPs, Hoffman noticed that the reduction potential (E Ared) of
PT
MMA (-1.1 V vs NHE) is more negative than the energy of CB (E CB) of ZnO (-0.2 V vs NHE). Thus, MMA should not be reduced by photo-excited electrons. It was proposed that stored
CC E
electrons with more negative redox potential (< -2.0 V), and thus potentially able to reduce MMA, may be localized at surface-trap sites or trapped under the form of Zn+ ion at the NP
A
surface [283]. Later investigations using annealed ZnO NPs in an unstirred solution pointed toward reaction details which differ from the polymerization in stirred reaction medium with non-annealed ZnO NPs [284]. In the former case, the polymerization reaction takes place preferentially at the NP surface, leading to a polymer-NP composite. Remarkably, unlike PMMA prepared by a conventional thermal radical process, the resulting PMMA product is predominantly syndiotactic with significant crystallinity [284]. 45
A disadvantage of the aforementioned ZnO and CdS NPs is their limited sensitivity to visible light [255,256,282,284,286–289]. The spectral sensitivity range was considerably expanded by using Fe2O3 NPs as photoinitiator to polymerize butyl methacrylate (BMA) in isopropanol solution under visible light [285,290]. As illustrated in Figure 31-b, since the ECB ≈ -0.45 V (vs NHE) of Fe2O3 is less negative than the EAred (-1.1 V vs NHE) of BMA monomer,
IP T
the direct transfer of the photoexcited electron from the CB to the monomer is not feasible.
However, isopropanol can be oxidized by the holes in the VB, leading to the formation of
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isopropoxy radicals, as the redox potentials of 2-propanol (EDox = -0.244 V vs NHE) is less positive than the energy of VB (E VB) of Fe2O3 [285]. Moreover, the protons formed during the oxidation of isopropanol may accept the excited electrons from the Fe2O3 CB to form active
U
hydrogen species (noted, for the sake of simplicity as H▪) which can further react with BMA or
N
alcohol molecules to generate radical initiating species. Thus, although the direct electron
A
transfer from the CB of the photoinitiator to the monomer molecule is thermodynamically
M
prohibited, the electron and hole generated upon the absorption of light may nevertheless lead
ED
to formation of monomer radicals capable of initiating a chain reaction [285]. The photopolymerization of MMA using CdS was reported by Popovic et al. who proposed
PT
a different mechanism based on the analysis of the thermal stability of the resulting PMMA [255,286]. Differential thermogravimetry (DTG) results show that the thermal stability of
CC E
PMMA synthesized with CdS photoinitiator was greater than PMMA thermally prepared with AIBN, but was comparable to PMMA prepared by anionic polymerization. The DTG curves of
A
the CdS initiated PMMA (Figure 32-a left) was composed of four peaks, three of which correspond to radical PMMA prepared with AIBN (Figure 32-a right). The additional peak of the photopolymerized PMMA suggested a CH2CH2 segment in position relative to a vinyl end-group, which is the signature of the initiation process [285]. Therefore, a plausible mechanism (Figure 32-b) was that the positive hole oxidizes a MMA molecule, leading to the formation of a proton and a very stable acrylic radical stabilized by -CO2Me. Addition of a 46
MMA unit on this stable radical leads to the formation of a propagating chain containing the CH2CH2 moiety in position of an unsaturated chain-end. It was suggested that these unsaturated chain-ends are thermally more stable than the conventional unsaturated chain ends arising from radical disproportionation, thus resulting in a photopolymerized PMMA with improved thermal stability [285].
in a non-degassed solution [291]. Using
1
IP T
The MMA photo polymerization reaction was performed using TiO2 NPs as photoinitiator
H Nuclear Magnetic Resonance (1H-NMR)
SC R
spectroscopy, the polymer was found to contain 6.9 % of isotactic, 38.1% of heterotactic and
54.9% syndiotactic triads. Such a microstructure is consistent with a radical PMMA. Interestingly, DTG analysis and theoretical calculations indicate that a fraction of the PMMA
N
U
chains possess the characteristic CH2CH2 moiety to an unsaturated chain-end (red area in Figure 32-c), implying an initiation mechanism similar to the CdS-MMA system (Figure 32-c,
M
A
(I)). In this study, due to the presence of dissolved O2 in the non-degassed liquid medium, O2•species can be formed by electron transfer from the CB of photoexcited TiO2 to O2, which then
ED
react with holes to generate the highly reactive OH • radical. These hydroxyl radical are known to initiate MMA radical polymerization (Figure 32-c, (II)), as observed in the case of a
PT
conventional MMA polymerization initiated by a redox Fe2+/H2O2 system. The generation of
CC E
OH• radicals was detected experimentally by Electron Spin Resonance (ESR) spectroscopy using dimethyl pyridine N-oxide (DMPO) as spin trap, thus confirming the proposed mechanism where both holes and OH • radicals work to generate initiating species [291].
A
The improvement of the semiconductor quantum efficiency is essential to enhance the
reaction efficiency [294,295]. After dye sensitization, the oxidation power is measured by the HOMO of the dye, which is usually less powerful as an oxidant than the electron hole [295]. Dye sensitized semiconductors, i.e., CdS and ZnO NPs, demonstrate significantly enhanced activity as initiators in photopolymerization [288,292]. Xanthene dyes in combination with ZnO 47
NPs are able to trigger the photopolymerization of BMA under visible light [288]. Eosin was used to sensitize CdS NPs for the photopolymerization of MMA [292], leading to a higher polymer yield in comparison to the system initiated with CdS NPs alone, or with CdS in combintation with triethylamine, NEt 3. Analysis by ESR spectroscopy confirmed that the CdS + eosin system generated more free radicals than the two other systems (Figure 33-b). The dye
IP T
sensitized CdS photoinitiator also showed good photostability for a long time because the dye
molecules were not photodegraded. It is believed that (Figure 33-a) upon light absorption, an
SC R
electron transfer occurs between the photoexcited dye (D*) and the CB of the semiconductor. The electrons in the CB band can then be transferred to the monomer molecules, leading to the
formation of an organic radical anion which can initiate polymerization [292]. Carbon doped
U
TiO2 also demonstrated visible / solar light activity to initiate the polymerization of MMA on
N
NP surface to form PMMA-grafted TiO2 nanostructure [296]. Compared with non-surface
A
confined PMMA, the photopolymerized PMMA confined at the surface the TiO 2 particle
M
possesses a high thermal stability as shown by TGA analysis. As shown by XRD, it is also
ED
crystalline, and its unit cell is unlike the one of syndiotactic PMMA which is obtained by low temperature radical polymerization. Hybrid polymer TiO2 nanocomposites obtained by
PT
photochemical grafting also exhibit interesting properties. For example, the in-situ polymerization of amine-containing 2-(tert-butylamino)ethyl methacrylate and ethylene glycol
CC E
dimethacrylate initiated by TiO2 NPs led to the formation of a core@shell TiO2@polymer nanocomposite which exhibits high antibacterial activity. This antibacterial activity is ascribed
A
to the combined action of the photocatalytic degradation by TiO 2 NPs, the biocidal polymeric amine containing polymer and the antifouling characteristic of the ethylene glycol units (Figure 33 c-f) [293]. Ravoo and coworkers developed a TiO2 NP photoinitiating system to carry out step-growth polymerization, which can be potentially be used for microcontact printing in a simple, rapid and economic way [297]. This technique took advantage of photoinduced electron / hole charge 48
carriers from semiconductors to induce reduction / oxidation of ethanol amine to initiate its step-growth polymerization. As illustrated in Figure 34, the photogenerated hole species of TiO2 oxidized the ethanol amine monomer (or the alcohol groups on the substrate surface) to the corresponding aldehyde, which reacts with another ethanol amine molecule to generate an imine. This imine was finally reduced by the photoexcited electrons of CB to form a secondary
IP T
amine, leading to the step growth polymerization of ethanol amine. A similar step-growth polymerization was also reported in the synthesis of polypyrrole using TiO2-based
SC R
nanostructures [298].
To increase the initiation efficiency, co-initiators, such as amines can be introduced to the photopolymerization system to promote the generation of free radicals as reported by Yagci et
U
al. when using ZnO NPs [5,299]. In degassed water, in the absence of co-initiators, the
N
photopolymerization of acrylamide can be initiated by ZnO alone due to the formation of OH •
A
radicals from the reaction of H2O / O2 with charge carriers (Figure 35-a). However, this reaction
M
is completely inhibited in organic medium. Co-initiators such as tertiary amines (TEA) and
ED
iodonium salt (Ph2I+PF6-) can be added to the system to trigger the photopolymerization of MMA in the absence of water. It is proposed that the lone pair on the N atom of TEA reacts
PT
with the photogenerated hole to form a radical cation, which can initiate the polymerization (Figure 35-b). In the case of the iodonium salt, a different route is proposed, consisting in the
CC E
direct reduction of the iodonium salt by the photogenerated electrons, as shown in Figure 35-c. Carboxylic acids can also work as co-initiator in photopolymerization [300,301]. Fe3O4 NPs
A
stabilized by lauric acid (LA) ligands (Figure 36-a) are capable of initiating the polymerization of MMA in the absence of any co-initiators, as the interaction of the photogenerated holes with the carboxylic acid groups leads to the decarboxylation of LA and the formation of free radicals, which can then initiate the polymerization (Figure 36-b). However, in the presence of TEA, αamino radicals can form and initiate the polymerization (Figure 36-c). A similar
49
decarboxylation process has also been reported during the photoinduced polymerization initiated by TiO2 NPs in the presence of butyric acid [302]. It is well-known that halide perovskites in optoelectronic applications can be in part attributed to their high absorption and long carrier lifetime, leading to a priming material for the application of photocatalysis and photovoltaics. Very recently, Tüysüz et al. [303] reported
IP T
a visible light driven in-situ photosynthesis of poly(3,4-ethylenedioxythiophene), PEDOT, over cesium lead iodide (CsPbI3) Perovskite QDs (Figure 37). Higher concentration of the CsPbI3
SC R
QDs increased polymerization rate, and after synthesis CsPbI3 QDs were encapsulated and stabilized by PEDOT to form a composite. During catalysis, 1,4-benzoquinone was used as the electron acceptor to promote polymerization, and to secure the desired cubic crystal structure
U
of CsPbI3 perovskite from oxidation. This novel approach enables a single-step synthesis of
A
3.1.2 Heteronanostructure or hybrid system
N
CsPbI3/PEDOT composite, which could be promising for optoelectronic applications.
M
Semiconductor-metal composite heteronanostructures have been widely used as
ED
photocatalyst for photosynthesis. Recently, ZnO@Ag composite nanostructures have been applied as photo-initiator to triger the free radical photopolymerization [302]. As shown in
PT
Fugure 38 a - c, ZnO@Ag heteronanostructure was prepared by depositing Ag NPs on ZnO nanoflowers using a redox active poly(ionic liquid), poly(1-vinyl-3-butylimidazolium
CC E
ascorbate) (P[VBuIm][As]). Under UV light irradiation, this material demonstrated good activity and stability (Figure 38 d and e) to initiate the polymerization of MMA and N-
A
isopropylacrylamide (NIPAM) in various solvents, such as water and dichloromethane (DCM), indicating a high versatility of this initiator. In comparison with ZnO nanoflower alone, the ZnO@Ag NPs showed much higher activity in photopolymerization, which can be ascribed to that the Ag NP on ZnO surface works as electron trap promoting the separation of photocharges as well as suppressing the recombination of electron-hole pairs at surface. Moreover, the authors proposed that the adsorbed H2O molecules on NP surface, due to the presence of highly 50
hygroscopic P[VBuIm][As], are critical for the generation of OH▪ radical species to initiate the polymerization, since no polymerization occurred if the reaction system was completely free from traces of moisture [303,304]. Besides inorganic semiconductor NPs, g-C3N4 can also be used as photoinitiators, due to their unique properties combining the characteristics of a middle-band semiconductor and
IP T
conductive graphite carbon with high surface area [305]. Indeed, g-C3N4 was found to be an active visible light catalyst, useful in water splitting, photdegradation of pollutants,
SC R
Friedel−Crafts reactions, reduction of CO2 and selective oxidation [305]. Yagci and co-workers reported that mesoporous g-C3N4 in the presence of alkyl amines as co-initiators can photoinitiate the free radical polymerization of MMA under visible light [305]. The initiation
A
N
being photooxidized in an initiating radical cation [299].
U
mechanism was presumed to be similar to the ZnO-TEA-MMA reaction system, with the amine
M
3.2 Controlled/Living Radical Polymerization
Despite the popularity and economic feasibility of free radical polymerization, the major
ED
limitation of this synthetic technique is the poor control over the synthetic process that usually
PT
generates wide polydispersity of molecular weight (Mw) and difficulty to achieve well-defined polymers with controlled composition, chain architecture, and site-specific functionality
CC E
[48,306]. Controlled / living radical polymerization (CRP) has revolutionized and revitalized the field of synthetic polymer chemistry over the last twenty years and it is now possible to prepare a wide variety of previously inaccessible macromolecules under relatively mild
A
conditions [48,306]. CRP realized the well-defined microstructure design of the polymers in a controlled fashion to synthesize bioconjugates, organic/inorganic composites, and surfacetethered copolymers with a narrow Mw polydispersity [48,306–308]. Several methods including transition-metal-catalyzed atom transfer radical polymerization (ATRP), reversible additionfragmentation chain transfer (RAFT) polymerization, single-electron-transfer living free radical 51
polymerization (SET-LRP) as well as nitroxide mediated polymerization (NMP) are increasingly well-developed CRP processes for the construction of macromolecular architectures [309]. Very recently photochemistry with CRP techniques has revolutionized for the synthesis of well-defined macromolecules [306]. In the following content, the ATRP and RAFT photopolymerizations upon photosensitive NP initiating system are highlighted.
IP T
In conventional ATPR synthesis, generally a transition metal complex catalyst, such as
Cu(I)-ligand complex, is necessary to establish the control over the polymerization
SC R
[136,306,311]. Alkyl halide compounds were used as initiators to react with transition metal catalyst to generate active initiating species in a reversible manner. In the example using cooper catalyst, the polymer growth highly depends on the redox cycle of Cu(I) / Cu(II), acting as a
U
reversible activation / deactivation process for initiation. An optimal control to the Cu(I) / Cu(II)
N
ratio is crucial to maximize the reaction rate while extending the lifetime of propagation chains
A
[299,310,312]. Liu et al. report the ATRP synthesis of several polymer brushes using TiO 2 as
M
the photo-reductant and Cu(II) complex as the catalyst to trigger the polymerization under UV
ED
irradiation via a one-electron transfer process [309]. Figure 39-a displays the mechanism of the UV-induced ATRP, where the air-stable Cu(II)-ligand was in situ reduced to active Cu(I)-
PT
ligand via accepting the photogenerated electrons from the CB of TiO2, which then reacted with the alkyl halide to produce alkyl radicals for initiating the polymerization; the Cu(I) can be
CC E
facilely recovered to Cu(II) by oxidation reaction with O 2 in air [309]. In this study, it is found that decreasing the Cu catalyst amount to ppm level did not retard the reaction significantly.
A
The living nature of this UV-ATRP synthetic process was verified by its ability to form copolymer brushes (Figure 39-b, c). Under irradiation, a new chain segment of poly(sulfopropyl methacrylate) (PSPMA) can be grafted onto the chain-end of the pre-synthesized poly(dimethylamino)ethyl methacrylate (PDEAEMA) to form a block polymer brushes (Figure 39-c) [309]. The break-off and re-start of the ATRP polymerization can be simply programmed by interval light on-off (Figure 36-d) [310]. Moreover, under irradiation, if successive monomer 52
was introduced to the reaction, the polymer chain can grew continuously (Figure 39-e). Both observations confirmed the living nature of the RAFT process with NP photoinitiator. The M w of the synthesized PMMA has a small polydispersity and the experimental polymer conversion ratio showed good correlation with theoretical calculation. Polymer brushes were also synthesized through the visible-light-driven ATRP synthesis in the presence of dye-sensitized
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TiO2 NPs and TiO2-rGO) nanocomposite [313,314]. The light induced ATRP allows the
application potential of micro-contact printing, where multi-functional groups with complex
SC R
architectures can be built / arranged orderly along the brush chain. On account of the same oneelectron transfer principle, ZnO, Fe-doped ZnO NPs, g-C3N4 NPs were also used as photo-
U
initiators in ATRP synthesis [299,310,311].
N
Recent research efforts have also been concentrated on developing new catalytic system
A
which can work efficiently under milder and environmentally benign conditions. For example,
M
instead of using toxic Cu, more biologically tolerable metal complexes including Ru and Fe at ppm level have been reported in ATRP synthesis [299,310,311], offering a “greener” reaction
ED
environment [5]. In addition, metal-free photoinitiated ATRP using semiconductors has recently reported by Liu and co-workers, where Nb(OH)5 NPs with benzyl alcoholate ligand
PT
(NbBA) were used as the visible light active initiator (Figure 40-a) [315]. In the presence of
CC E
NbBA and Ethyl 2-bromoisobutyrate (EBiB) as catalyst, the ATRP synthesis of poly(isopropylacrylamide) (PNIPAAm) was initiated under visible light. The proposed mechanism was illustrated in Figure 40-b, upon the photoexcitation, electron transfer occurs
A
via ligand-to-metal charge transfer process from the surface complex ligand to the metal atom of the NP, leading to the formation of radical cations of the ligand followed by the generation of a –O-CH•-Ph radical after losing the α-hydrogen. The resulted alkoxy substituted radical is highly active to react with the EBiB catalyst to yield the alkyl radical to initiate polymerization. As presented in Figure 40-a, the regeneration of NbBA, homolysis of the C-Br bond, chain 53
propagation can take place repeatedly in the cyclical process. Without using EBiB catalyst, this reaction undergoes a solely free radical polymerization process with poor control over M n and polydispersity. The NPs can be easily separated by centrifuge and reused for successive reactions. Another advantage is that this metal free reaction system enables the producing of clean products without additional efforts to remove the ionic metal residues [315].
IP T
Compared with the ATRP synthesis, RAFT polymerization renders a metal ion-free environment, which is promising for biological relevant applications. The NbBA NPs were also
SC R
successfully applied in visible light-driven RAFT polymerization, in which PNIPAAm with controlled Mn and narrow polydispersity index was obtained if the feeding ratio of NP to RAFT
agent was controlled properly [316]. Like the mechanism proposed in the above ATRP case, in
U
RAFT synthesis, the photoinduced –O–CH•–Ph radical can attack the C=S bond of the RAFT
N
agent to generate the highly reactive intermediate, initiating the RAFT polymerization (Figure
A
40-c). Afterward, C=S bond of the RAFT agent can be reformed with a radical fragment by the
M
decomposition of the radical intermediate. The raidical fragment can initiate the chain
ED
propagation and subsequently react with the NbBA-RAFT to produce a highly active polymeric intermediate again (f), which could decompose into the polymer with RAFT active center (g)
PT
and NbBA NP for the following repeating reactions. Like the ATRP process, the wellcontrolled manner of this photo-induced ATRP was evidenced by light on-off test (Figure 40-
CC E
d). The living nature was confirmed by a continuous growth of the polymer chain upon the introduction of additional monomers under irradiation (Figure 40-e) [316].
A
3.3 Free Radical Photopolymerization Induced by Other NPs Apart from the semiconductors, other photo-responsive nanostructures have been developed to assist the radical polymerization. Lanthanide-doped upconverting NPs (UCNPs) have revealed profound potential as fluorescent probes in bio-applications, such as bio-imaging and bioassays [317]. In particular, UCNPs is able to transfer low-energy near-infrared (NIR) incident photons 54
to high-energy UV or visible emission based on the sequential absorption of two or more photons. NIR light serving as the excitation source offers a distinct advantage for bio-system, as it enables higher penetration depth in tissues without photo-damage to living cells. Haupt et al. [318] reported generic strategy to construct a cross-linked polymer coating with plenty of optional functional groups on UCNPs. As illustrated in Figure 41, upon the NIR (λ = 980 nm)
IP T
irradiation, the UV emission was generated by the UPCNs, which was then captured by the dye molecules to generate active species [318]. Co-initiators were activated by the excited dye to
SC R
generate free radicals initiating the polymerization. Since the emission from the UCNPs is weak as compared to the direct light, the polymerization is proceeded only at the confined, close proximity of the UCNP surface, and thus resulting in the formation of a UNCP@polymer
U
core@shell structure. By simply varying the monomers, polymer coating with various
N
properties can be programmed facilely, which can be hydrophilic or hydrophobic, bio-
A
compatible or sensitive, chemically reactive or inert. Multilayer polymeric coating was also
M
achievable by sequentially introducing different monomers with variety of functionalities [318].
ED
SiO2 NPs are considered to be non-toxic, biocompatible materials with unique surface features, such as the rich of –OH groups, high porosity as well as large surface area, which
PT
render them to be a versatile platform for surface functionalization to meet various applications. Ruthenium complex (RuL2+) was immobilized in a Nafion (Nf) coating wrapped on SiO2 NP
CC E
to form a visible-light-active hybrid catalyst (SiO2/Nf/RuL) for radical polymerization (Figure 42-a) [319]. Due to the presence of a great number of negatively charged sulfonate groups in
A
Nafion, RuL2+ can be bound to surface of the SiO2@Nf structure, attributed to a strong electrostatic attraction between RuL2+ and Nifion (Figure 42-b). In the presence of the SiO2/Nf/RuL, free radical polymerization of MMA can be performed at ambient temperature under visible light irradiation using EBiB as initiator. As shown in Figure 42-c, the RuIIL confined in the Nifion coating was excited to Ru*L upon photon absorption, which is reduced by an electron donor, iso–dipropylethylamine (iPr2NEt), via electron transfer process, to form 55
a highly active single electron reductant RuIL. The interaction of RuIL and EBiB initiator produced a carbon-centered radical for polymerization and reduced RuIIL back to RuIL again in the photoredox cycle [319]. Schlögl and co-workers developed photoactive SiO2 corephotocleavable shell NPs by immobilizing bis(acyl)phosphane oxide and aromatic αhydroxyketone derivatives on SiO2 NP surface using silanization method, which can efficiently
IP T
undergo Norrish type I photofragmentation reactions [320]. They studied the photo-activity of these photoactive core-shell structures as photo-initiators for free radical polymerization of
SC R
thiol-ene. Results revealed that the photoactivity of the NPs is not only influenced by the type
of initiating species of the organic shell but also by the grafting method used for NP synthesis, since the latter can control the size, dispersion and surface functionalization of the NPs [320].
U
Metal NP-polymer composites have attracted increasing research interest for the
N
development of functional materials with unique electrical, optical or mechanical properties in
A
the application of optics, bio-imaging and sensors [321,322]. In particular, Au NPs have been
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an intense research topic due to their fascinating properties and catalytic activity in various
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reactions [323–325]. Neckers and co-workers studied the polymerization of acrylates using 5Mercapto-2,2-bithiophene (BTS) capped Au NPs under UV and visible light irradiation, where
PT
the function of the Au was highly dependent on the excitation light [326,327]. Upon exposure to the UV irradiation (λ = 350 nm, corresponding to BTS absorption), the BTS ligands on Au
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surface were excited to generate highly active radical cations at the thiophene rings of Au NPBTS+• via electron transfer to the conductive Au NP, which consequently became the unstable
A
acryl radical through deprotonation to initiate the polymerization (Figure 43-a). As the polymerization was only taken place in a close vicinity to the NP surface, which strictly localized reaction largely reduced the rate of product diffusion. On the other side, visible light irradiation (λ > 450 nm, Figure 43-b) led to different photo-radical generation mechanism due to the SPR effect of Au [326]. Au NPs can act as electron donors upon excitation at the SPR band to generate active surface species, although BTS is not light-responsive at visible region. 56
It is suggested that on account to the excitation of Au NP SPR in the presence of an oxidizing chemical of [4-[(octyloxy)phenyl] phenyl] iodonium hexafluoroantimonate (OPPI) as coinitiator, electron was transferred from the Au NP to OPPI inducing the decomposition of OPPI to form phenyl radicals for polymerization. This proposal was evidenced by the fact that without the presence of monomers, intraparticle coupling took place though BTS ligand connection to
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form NP aggregates via intramolecular electron transfer and subsequent charge recombination.
Baymberg et al. reported SPR-induced radical polymerization via plasmonic “hot”
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electrons using Au NPs [328]. As illustrated in Figure 43-c, Au NPs were deposited on a Au
“mirror” surface with tunable gap, where strong electric fields were formed due to the coupling of Au substrate and SPR. Under visible light (laser) illumination, N-isopropylacrylamide
U
(NIPAM) polymer growth was triggered around the junction of the Au NP and substrate,
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without using any additional initiator. It is proposed that upon the absorption of visible photons,
A
due to the strong localized SPR electric fields, hot-electron transfer takes place from Au NP
M
surface to monomers to form initiating free radicals for polymer growth, demonstrating the
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radical polymerization initiated by light-directed programmable plasmons [328,329].
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3.4 Reaction Kinetics in Polymerization According to classic kinetic theory of a conventional free radical polymerization reaction with
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a given monomer concentration, the overall rate of polymerization, Rp, is given by the following equation [47]:
𝑑 [𝑀 ] 1/2 = 𝑅𝑝 = 𝑘𝑝 𝑘𝑡 [𝑀]𝑅𝑖𝑛 1/2 𝑑𝑡
A
−
where Rin is the initiation rate, [M] is the monomer concentration; kp and kt are the rate constants of propagation and termination, respectively, which can be referred from literature values. Clearly, in a given monomer system, Rp is proportional to the square root of the initiation rate, indicating that the reaction ratio is mainly dominated by the initiation step. In the presence of a
57
photoinitiator, the initiation process is the only photochemical step. The chain initiation rate can thus be determined from the following expression [256,282]: 𝑅𝑖𝑛 = Q 𝑖 𝐼𝑎 = Q 𝑖 𝐼0 (1 − 10−𝐴 ) where Qi is the photoinitiation quantum yield, A presents the absorbance of the NPs in solution, I0 and Ia are the incident and the absorbed light intensity. Once the monomer radical is formed,
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subsequent chain propagation will comply with the classic free radical chain growth mechanism. Hence, the overall rate of a free radical polymerization under the steady state using NP
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photoinitiator can be expressed as follows [256,282]:
1/2
1/2 𝑅𝑝 = 𝑘𝑝 𝑘𝑡 [𝑀](Q 𝑖 𝐼0 (1 − 10−𝐴 ))
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Therefore, with regard to the materials, quantum efficiency is a crucial factor significantly
N
determining the rate of photo-polymerization. NP photoinitiators with high quantum efficiency
A
and good charge separation feature are of large benefit to the photopolymerization. The kinetic
M
nature of photopolymerization has been investigated experimentally via varying the reaction parameters, such as irradiation time, intensity of incident light, monomer concentration and NP
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concentration. In most cases, the monomer consumption ([M]/[M0]) basically follows a firstorder dependence of the polymerization rate as shown in Figure 44-a [136]. The polymer yield
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increases linearly with the extension of irradiation time, indicating that the rate of
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polymerization was independent of time (Figure 44-b) [284]. The plot of the polymerization rate versus the square root of light intensity is linear, which is in perfect agreement with the kinetic theory shown above (Figure 44-c) [256]. Besides the quality of the NPs, monomer,
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solvent or co-initiator also may influence the reaction (Figure 44-d). For example, in an early report of Hoffman, the polymerization efficiency of vinyl acetate, methyl acrylate, acrylonitrile, MMA and styrene was assessed using a CdS quantum dot photo-polymerization system. It is found that besides the reduction potential of the monomers, their intrinsic reactivity and polarity played more essential roles in the reaction [282]. The relationship between quantum yield of 58
NPs and dielectric constant of the solvent was also discussed, but such solvent effect may not applicable to other reaction system [282]. Nonetheless, usually the solvent with reduction potential compatible with the energy level of semiconductor NPs, which may work as a good hole scavenger to promote the charge separation, is preferred. Moreover, the choosing of coinitiators needs to be carefully considered. The co-initiators can dramatically enhance the
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efficiency of the polymerization, as they may interact with the surface of the NPs, where photoexcited charge carriers are inherently located [300,329]. The effect of various amine co-
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initiators on the one- and two-photon induced polymerization of MMA using CdS QDs implied that some amines, i.e. TEA, can interact more strongly with trapped carriers than other amines (such as n-butylamine and diethylethlenediamine), although the nature of the interaction is still
U
unknown [329].
A
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4 Conclusion and Perspective
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In line with the concept of “green” and sustainable chemistry, photocatalytic organic synthesis, e.g. photocatalytic conversion of CO2, aromatic alcohols, fatty alcohols and other compounds
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to fuel, organic molecules and polymers, as one important branch and a frontier in photosynthesis has opened a promising and environmental-friendly venue for the production of
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organics. In this paper, we mainly highlighted the most recent advances about the CO2
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conversion, organic molecule transformation, and photopolymerization initiated by photoactive NPs and. The introduction of photocatalytic CO2 reduction is categorized depending on the catalyst types; and at the end of the introduction we briefly discussed several important
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factors affecting catalysis. The section of photocatalytic organic transform is classified based on the reaction types, and ended with a summary of factors influencing reaction selectivity. In the third part, for the first time, we summarized the application of photoresponsive NPs, which have long been used as photocatalysts in heterogenous catalysis, in the radical polymerization system working as photo-initiators. The semiconductor NPs or nanocomposites are good light59
sensitive materials undergoing photon adsorption followed by the generation of charge carriers, which can induce the formation of initial radicals. The possible reaction mechanism is highly dependent on the type of the NPs, reactants and solvents. Compare with the conventional heterogenous catalysis system, the organic synthesis and photopolymerization initiated by NPs is more complex since monomers, solvents and co-initiators all participate the reaction. In
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addition to the good activity, several outstanding merits of using NP photoinitiator, including,
but not limited to the mild reaction conditions, products with high purity related to the easy
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separation of NPs, and facilely recyclable / reusable NP initiators, make them highly attractive for many applications.
Despite the achievements over the past decades, there are many challenges still existing in
U
the domain of photocatalytic organic synthesis. The photoconversion efficiency and selectivity
N
for desired products are lower than photocatalytic H 2 evolution or natural photosynthesis. In
A
addition to continue developing of highly active catalyst, the exact mechanism needs to be
M
systematically investigated, in particular the adsorption and activation of reactants on catalyst
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surface which are playing key role affecting reaction rate and selectivity. Other processing factors determining the reaction selectivity need to be systematically studied. The investigation
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using nanomaterials for photopolymerization is still largely limited in comparison to those in heterogenous catalysis. To further expand the application of photoresponsive nanomaterials in
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radical polymerization, more efforts need to be devoted to the following aspects: Firstly, it is critical to develop more active NP photoinitiators. Regarding the materials, although classic
A
semiconductors show good performance in photopolymerizations, there is a large ascendant space to develop high performance photoresponsive nanostructures, such as larger surface area, broader spectral absorption, higher quantum efficiency as well as better charge separation. Nanocomposites containing semiconductors, metal NPs or carbon materials with rationally designed architectures may be promising solutions. Also, it is timesaving but interesting to transfer the recently developed novel nanocatalysts from heterogenous catalysis into the 60
photopolymerization system to assess the reaction efficiency and explore the possible mechanism. Secondly, the radical polymer synthetic system needs to be extended. In addition to increase the study on the controlled / living radical polymerization, NP photoinitiator may also be applied in microemulsion or reverse-micelle polymerization, where more detailed study can be carried out in terms of the effect of surfactant and additives on the photoinitiation. Third,
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co-initiators, such as alkyl alcohol or amines, play an important role to increase the reaction efficiency, as they can react directly with the separated electrons and holes to form active
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species. Therefore, establishment of an optimal partnership between co-initiator and NP photoinitiating dynamics which can favor the reaction synergistically requires numerous attempts. Meanwhile, the influence of solvent on photopolymerization cannot be neglected. The
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last but not least, in the regard of the universality of radical polymerization in practical
N
manufacturing, polymerization reactions using a larger variety of monomers are expected. In
A
previous study, mainly acrylate chemicals (e.g., MMA) were used as the representative
M
monomer only to display the feasibility of the polymerization; however, it is far away from the
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development of the application of the NP photopolymerization system. Therefore, in the future, this may be improved by using nano-photoinitiators to synthesize macromolecules, polymers /
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copolymers with more functionalities and more complex structures.
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Acknowledgements
All authors would like to acknowledge research funding of National Key R&D Program of China (2017YFE0102700) from the Ministry of Science and Technology (MOST). J.Z. is
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grateful for the support from National Natural Science Foundation of China (NSFC, No. 51872128) and Jiangsu University (17JDG008). H.L. thanks the support from NSFC (No. 51732007). H-N.L. thanks the NSFC (No. 21705085) and Jiangsu University (17JDG007). References
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Figure 1. Schematic illustration of the natural photosynthesis (left) and an artificial heterogenous photosynthesis using semiconductor NP as catalyst (right) [13]. Reprinted with
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Figure 2. Proposed mechanism and pathways for the photocatalytic reduction of CO2 with H2O
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vapor to CH3OH and CH4 using TiO2 [49]. Reprinted with permission.
Figure 3. Amount of liquid phase products with time and the schematic consecutive pathway
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Figure 4. (a) Transmission electron microscopy (TEM) and high resolution TEM (HR-TEM)
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images of TiO2 nanocubes with single crystalline and exposed {100} and {001} facets. (b) Evolution rate of CH4 and CH3OH using TiO2 nanotubes and other TiO2 catalysts [60]. (c)
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Scanned electron microscopy (SEM) images of the hexahedron prism-anchored octahedronal CeO2 with different prism length. (d) HR-TEM images of the prism. (e) CH4 evolution using
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different CeO2 catalysts. (f) Proposed PET process for photocatalysis using hexahedron prism-
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anchored octahedronal CeO2 [61]. Reprinted with permission.
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Figure 5. (a) Methanol production as a function of reaction time upon different photocatalysts. (b) Plausible mechanism of CO2 reduction on in situ Ru(bpy)3/TiO2 nanocomposite [94].
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Figure 6. Scheme of the two different mechanisms for charge carrier separation: (a)
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conventional heterojunction-type and (b) direct Z-scheme mechanisms [96]. Reprinted with
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Figure 7. (a) Scanned electron microscopy (SEM) image of the SiC@MoS2 nanoflowers. (b) CH4 evolution on the photocatalyst SiC@MoS2 with diferent MoS2 content for 4h reaction. (c) Proposed reaction pathway of photocatalytic CO2 reduction with H2O and SiC@MoS2 [158]. Reprinted with permission
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Figure 8. (a-f) TEM images of the Bi2 WO6-TiO2 binanosheets. (g) Proposed Z-scheme process
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of the photocatalytic CO2 reduction upon the binanosheets [145]. Reprinted with permission.
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Figure 9. Proposed structure modes of CO2 bonding on catalyst [25,191–193]. Reprinted with permission.
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Figure 10. Selective oxidation of p-aminobenzyl alcohol into p-aminobenzaldehyde over
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Au/CeO2 catalyst under green light irradiation [218]. Reprinted with permission.
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Figure 11. HR-TEM images of the Au NPs loaded on the interface of anatase/rutile TiO2. And the proposed process of oxidation of aromatic alcohols on the interface of mixed crystal
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structure [222]. Reprinted with permission
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Figure 12. (a) Reaction scheme of the photoenzymatic hydroxylation of ethyl benzene [223]. (b) Schematic presentation for the synthesis of noble-metal@TiO2-microspheres [225].
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Figure 13. Schematic illustration of the self-assembled heterosupramolecular visible light
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photocatalyst [226]. Reprinted with permission.
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Figure 14. (a) SEM image of the p-BWO nanosheets. (b) Reversible change of p-BWO appearance upon light and O2 presence. (c) Toluene conversion and yields of benzyl alcohol, benzaldehyde and benzoic acid. (d) Conversion rate of toluene oxidation for Bi2WO6, and p-
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BWO at different catalyst loadings [227]. Reprinted with permission.
Figure 15. Aromatic alcohol oxidation over Au-Pd alloy NPs under visible light irradiation
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[229]. Reprinted with permission.
Figure 16. Schematic illustration of the fabrication of CdS(GRM) (M = Ca2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) nanocomposites in which metal ions are introduced to the interfacial layer matrix between GR and semiconductor CdS [233]. Reprinted with permission
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Figure 17. (a) Schematic illustration of the routes to fragment β-O-4 linkage for conversion of lignin. Route A, traditional biorefinery process forming cellulose pulp and technical lignin.
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Route B, lignin-first approach with high-temperature hydrogenolysis upon a conventional thermal catalysis process using supported metal catalyst. Route C, lignin-first approach with
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solar energy-driven photocatalytic process under mild conditions. (b) Scheme of the lightdriven lignin-first valorization of birch woodmeal using CdS QDs as photocatalyst [237]. Reprinted with permission.
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Figure 18. (a, b) TEM images of the WS2/Pd nanohybrids. (c) Photocatalytic Suzuki C-C
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coupling reaction over WS2/Pd and proposed reaction mechanism [238]. Reprinted with
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Figure 19. (Upper) TEM and HR-TEM images of the CdS-MoS2 catalyst and corresponding elemental distribution. (Lower) Schematic illustration of reaction energy profiles via ⋅CH2OH and CH3O⋅ on CdS(100) [239]. Reprinted with permission 95
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Figure 20. (a) SEM images of the nanostructure of Pt@MoO on BVO. (b) EG conversion
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efficiency of the catalysts under visible light irradiation. (c) Proposed mechanism for the
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photocatalytic synthesis of EG over Pt@MoO/ BVO [162]. Reprinted with permission.
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Figure 21. (a, b) TEM and HR-TEM images of the Cu@TiN catalyst. (c) Schematic illustration of the reaction process of the photocatalytic epoxidation of alkenes using Cu NPs as catalyst
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[241]. Reprinted with permission.
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Figure 22. Scheme of reactions of synthesis secondary amines using primary amines as starting materials upon Pt/TiO2 photocatalyst, and a summary of the products (chart) [245]. Reprinted
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Figure 23. Proposed reaction mechanism for the photo-alkylation of aniline over Au–TiO2
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[249]. Reprinted with permission.
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Figure 24. Schematic illustration of the photocatalysis promoted 4-NP reduction into 4-NP
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under visible (a) and UV light (b) irradiation [254].
Figure 25. Photocatalytic formation of imines using TiO2 as catalyst and nitrobenzene as reactant [255]. Reprinted with permission.
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Figure 26. Schematic illustration of quinolines synthesis from nitroaromatic compounds and
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alcohols using arenesulfonic acid functionalized SiO2-TiO2 photocatalyst [262]. Reprinted with
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Figure 27. Ru NP size (left: small and unsupported NPs) and support effect (righ: large and
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supported NPs) on self-coupling of primary amines [264]. Reprinted with permission.
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Figure 28. Schematic illustration of selective photocatalytic oxidations and reductions of
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mixtures over templated nanocavities [275]. Reprinted with permission.
Figure 29. Amount of products and nitrobenzene selectivity during photocatalytic reaction of aniline
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using Pt/P25 and proposed mechanism of selective oxidation of aniline on Pt/P25 under visible light irradiation [278]. Reprinted with permission.
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Figure 30. Photocatalytic oxidation of benzene and alkylbenzenes by Pt/TiO2 photocatalysts using
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incident light with different wavelengths [279]. Reprinted with permission.
Figure 31. Schematic illustration of photoinduced MMA and BMA polymerization using ZnO (or CdS) [280,281] (a) and Fe2O3 (b) NPs [285], respectively. Reprinted with permission.
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Figure 32. (a) Deconvolution of the DTG curves of PMMA prepared using CdS (left) and AIBN (right). (b) Scheme of the generation of monomer radical using CdS [285]. (c) Two
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possible radical generation mechanisms using TiO 2 [291]. Reprinted with permission.
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Figure 33. (a) Scheme of radical generation and chain growth using dye-sensitized CdS. (b) EPR spectra of different semiconductor system [292]. (c) Transmission electron microscopy (TEM) images of the TiO2 / poly(TBAM-co-EGDMA) core/shell NPs. (d) Bacteria survival (%) after treatment with pristine TiO2, bulk poly(TBAM-co-EGDMA), and TiO2/poly(TBAM-coEGDMA) nanoparticles in the presence and the absence of UV light irradiation. Bacteria
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survival ratio (%) vs. UV-irradiation time on the surface of pristine TiO2 and TiO2/poly(TBAM-
co-EGDMA) nanoparticles against (e) Escherichia coli and (f) Staphylococcus aureus [293].
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Figure 34. (a) Scheme of the photocatalytic polymerization by printing of ethanol amine with TiO2 on a 11-(tricholorosilyl)undercan-1-ol self-assembled monolayer. (b) Atomic force microscope (AFM) image of the patterned PEI brushes. (c) Proposed reaction mechanism of the photocatalytic step-growth of ethanol amine [297]. Reprinted with permission. 103
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Figure 35. Interaction of the photoexcited electron and hole species with different co-initiators
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in a ZnO initiating system [299]. Reprinted with permission.
Figure 36. (a) Illustration of the LA coated Fe3O4 and the amine co-initiator. Proposed mechanism for the photopolymerization of vinyl monomers using LA-Fe3O4 NPs without (b) and with co-initiator (c) [301]. Reprinted with permission.
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Figure 37. Schematic illustration of PEDOT synthesis using CsPbI 3 as the photoinitiator [303].
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Figure 38. (a) Synthesis of ZnO@Ag composite heteronanostructure using P[VBuIm][As]. (b, c) SEM images of ZnO nanoflowers and ZnO@Ag NPs. (d) Conversion ratio of MMA polymerization using ZnO@Ag NPs as photoinitiator. (e) Continuous cycles of polymerization
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using ZnO@Ag NPs [302]. Reprinted with permission.
Figure 39. (a) Photoinduced ATRP using semiconductor NPs as photoinitiators [6]. (b)
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Thickness of the polymer brushes obtained from various monomers after 1 h ATRP reaction. (c) Left figure shows the line traces of the PDMAEMA and PSPMA-PDMAEMA brushes AFM images. The right image is the schematic illustration of the PSPMA=PDEAEMA multilayer
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brushes produced by UV-ATRP [309]. (d) UV-light on-off test during the ATRP of MMA. (e) Gel permeation chromatography (GPC) of the PMMA during chain extension process before (black) and after (gray) sequential monomer addition [310]. Reprinted with permission.
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Figure 40. (a) Scanned electron microscopy (left) and TEM (right) image of NbBA NPs. (b)
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Proposed mechanism of a visible light driven ATRP (b) and RAFT (c) polymerization using
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NbBA as the photoinitiator. (d) Light on-off test of RAFT polymerization of NIPAAm using
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NbBA as photoinitiator. (e) GPC traces of PNIPAAm-b-PDMA (dashed line) and PNIPAAm
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(macro-CTA, solid line) to confirm the living character test of RAFT polymerization using
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NbBA photoinitiator [315,316]. Reprinted with permission.
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Figure 41. Scheme of the UCNP assisted photopolymerization (a) in the presence of UV and
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visible light photoinitiators (b), and the detailed radical formation process (c) [318]. Reprinted
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Figure 42. (a) Structure scheme of the SiO2/RuL and SiO2/Nf/RuL. (b) Dye adsorption ability of SiO2/RuL and SiO2/Nf/RuL. (c) Scheme of the photopolymerization using SiO 2/Nf/RuL as
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photoinitiator [319]. Reprinted with permission.
Figure 43. Schematic mechanism of the photopolymerization under UV (a) and visible light (b)
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using light responsive Au NP-BST [326,327]. TEM images show the Au NP-BST before and after light irradiation without monomers [326]. (c) Scheme of the plasmon initiated
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polymerization and the scanned electron microscopy (SEM) images of Au NPs before and after
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irradiation, showing the formation of polymer [328]. Reprinted with permission.
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Figure 44. (a) Monomer consumption (ln([M]/[M0])) vs. reaction time [136]. (b) polymer yield
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as a function of irradiation time [284]. (c) Polymerization rate vs. Square root of light intensity
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[256]. (d) Co-initiator effect on the polymerization [329]. Reprinted with permission.
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